feat(auth): add Bucket4j + Caffeine login rate limiter (10/15 min per IP+email, 20/15 min per IP)

LoginRateLimiter uses two Caffeine LoadingCaches of Bucket4j buckets —
one keyed on IP:email (10 attempts/15 min) and one on IP alone (20/15 min
backstop). Exceeding either throws DomainException(TOO_MANY_LOGIN_ATTEMPTS)
and emits LOGIN_RATE_LIMITED audit. Successful login invalidates both
buckets via invalidateOnSuccess. Buckets expire after windowMinutes of
inactivity (no clock advance needed — Caffeine handles eviction).
AuthService integrates it as an optional @Autowired field so non-web
test contexts still work without a Caffeine dependency.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/pom.xml

@@ -180,11 +180,16 @@
 			<artifactId>flyway-database-postgresql</artifactId>
 		</dependency>
 
-		<!-- Caffeine cache for in-memory rate limiting -->
+		<!-- Caffeine cache + Bucket4j for in-memory rate limiting -->
 		<dependency>
 			<groupId>com.github.ben-manes.caffeine</groupId>
 			<artifactId>caffeine</artifactId>
 		</dependency>
+		<dependency>
+			<groupId>com.bucket4j</groupId>
+			<artifactId>bucket4j-core</artifactId>
+			<version>8.10.1</version>
+		</dependency>
 
 		<!-- OpenAPI / Swagger UI — enabled only in the dev Spring profile -->
 		<dependency>

backend/src/main/java/org/raddatz/familienarchiv/auth/AuthService.java

@@ -30,12 +30,25 @@ public class AuthService {
     @Autowired(required = false)
     private JdbcIndexedSessionRepository sessionRepository;
 
+    @Autowired(required = false)
+    private LoginRateLimiter loginRateLimiter;
+
     /**
      * Validates credentials and returns the authenticated user plus the Spring Security
      * Authentication object. The caller is responsible for persisting the Authentication
      * to the session via SecurityContextRepository.
      */
     public LoginResult login(String email, String password, String ip, String ua) {
+        if (loginRateLimiter != null) {
+            try {
+                loginRateLimiter.checkAndConsume(ip, email);
+            } catch (DomainException ex) {
+                auditService.log(AuditKind.LOGIN_RATE_LIMITED, null, null, Map.of(
+                        "ip", ip,
+                        "email", email));
+                throw ex;
+            }
+        }
         try {
             Authentication auth = authenticationManager.authenticate(
                     new UsernamePasswordAuthenticationToken(email, password));
@@ -45,6 +58,9 @@ public class AuthService {
                     "userId", user.getId().toString(),
                     "ip", ip,
                     "ua", truncateUa(ua)));
+            if (loginRateLimiter != null) {
+                loginRateLimiter.invalidateOnSuccess(ip, email);
+            }
             return new LoginResult(user, auth);
         } catch (AuthenticationException ex) {
             // Audit login failure — intentionally does NOT log the attempted password.

backend/src/main/java/org/raddatz/familienarchiv/auth/LoginRateLimiter.java

@@ -0,0 +1,61 @@
+package org.raddatz.familienarchiv.auth;
+
+import com.github.benmanes.caffeine.cache.Caffeine;
+import com.github.benmanes.caffeine.cache.LoadingCache;
+import io.github.bucket4j.Bandwidth;
+import io.github.bucket4j.Bucket;
+import lombok.extern.slf4j.Slf4j;
+import org.raddatz.familienarchiv.exception.DomainException;
+import org.raddatz.familienarchiv.exception.ErrorCode;
+import org.springframework.stereotype.Service;
+
+import java.time.Duration;
+import java.util.concurrent.TimeUnit;
+
+@Service
+@Slf4j
+public class LoginRateLimiter {
+
+    private final LoadingCache<String, Bucket> byIpEmail;
+    private final LoadingCache<String, Bucket> byIp;
+    private final int maxPerIpEmail;
+    private final int maxPerIp;
+    private final int windowMinutes;
+
+    public LoginRateLimiter(RateLimitProperties props) {
+        this.maxPerIpEmail = props.getMaxAttemptsPerIpEmail();
+        this.maxPerIp = props.getMaxAttemptsPerIp();
+        this.windowMinutes = props.getWindowMinutes();
+
+        this.byIpEmail = Caffeine.newBuilder()
+                .expireAfterAccess(windowMinutes, TimeUnit.MINUTES)
+                .build(key -> newBucket(maxPerIpEmail, windowMinutes));
+
+        this.byIp = Caffeine.newBuilder()
+                .expireAfterAccess(windowMinutes, TimeUnit.MINUTES)
+                .build(key -> newBucket(maxPerIp, windowMinutes));
+    }
+
+    public void checkAndConsume(String ip, String email) {
+        boolean ipEmailOk = byIpEmail.get(ip + ":" + email).tryConsume(1);
+        boolean ipOk = byIp.get(ip).tryConsume(1);
+        if (!ipEmailOk || !ipOk) {
+            throw DomainException.tooManyRequests(ErrorCode.TOO_MANY_LOGIN_ATTEMPTS,
+                    "Too many login attempts from " + ip);
+        }
+    }
+
+    public void invalidateOnSuccess(String ip, String email) {
+        byIpEmail.invalidate(ip + ":" + email);
+        byIp.invalidate(ip);
+    }
+
+    private static Bucket newBucket(int limit, int minutes) {
+        return Bucket.builder()
+                .addLimit(Bandwidth.builder()
+                        .capacity(limit)
+                        .refillGreedy(limit, Duration.ofMinutes(minutes))
+                        .build())
+                .build();
+    }
+}

backend/src/main/java/org/raddatz/familienarchiv/auth/RateLimitProperties.java

@@ -0,0 +1,14 @@
+package org.raddatz.familienarchiv.auth;
+
+import lombok.Data;
+import org.springframework.boot.context.properties.ConfigurationProperties;
+import org.springframework.stereotype.Component;
+
+@Component
+@ConfigurationProperties("rate-limit.login")
+@Data
+public class RateLimitProperties {
+    private int maxAttemptsPerIpEmail = 10;
+    private int maxAttemptsPerIp = 20;
+    private int windowMinutes = 15;
+}

backend/src/main/resources/application.yaml

@@ -150,3 +150,9 @@ sentry:
   enable-tracing: true
   ignored-exceptions-for-type:
     - org.raddatz.familienarchiv.exception.DomainException
+
+rate-limit:
+  login:
+    max-attempts-per-ip-email: 10
+    max-attempts-per-ip: 20
+    window-minutes: 15

backend/src/test/java/org/raddatz/familienarchiv/auth/AuthServiceTest.java

@@ -16,6 +16,7 @@ import org.springframework.security.authentication.BadCredentialsException;
 import org.springframework.security.authentication.UsernamePasswordAuthenticationToken;
 import org.springframework.security.core.Authentication;
 import org.springframework.session.jdbc.JdbcIndexedSessionRepository;
+import org.raddatz.familienarchiv.exception.ErrorCode;
 
 import java.util.HashMap;
 import java.util.Map;
@@ -37,14 +38,16 @@ class AuthServiceTest {
     @Mock UserService userService;
     @Mock AuditService auditService;
     @Mock JdbcIndexedSessionRepository sessionRepository;
+    @Mock LoginRateLimiter loginRateLimiter;
     @InjectMocks AuthService authService;
 
     private static final String IP = "127.0.0.1";
     private static final String UA = "Mozilla/5.0 (Test)";
 
     @BeforeEach
-    void injectSessionRepository() {
+    void injectOptionalFields() {
         ReflectionTestUtils.setField(authService, "sessionRepository", sessionRepository);
+        ReflectionTestUtils.setField(authService, "loginRateLimiter", loginRateLimiter);
     }
 
     @Test
@@ -141,6 +144,45 @@ class AuthServiceTest {
         );
     }
 
+    @Test
+    void login_checks_rate_limit_before_authenticating() {
+        doThrow(DomainException.tooManyRequests(ErrorCode.TOO_MANY_LOGIN_ATTEMPTS, "rate limited"))
+            .when(loginRateLimiter).checkAndConsume(IP, "user@test.de");
+
+        assertThatThrownBy(() -> authService.login("user@test.de", "pass", IP, UA))
+            .isInstanceOf(DomainException.class)
+            .satisfies(ex -> assertThat(((DomainException) ex).getCode())
+                .isEqualTo(ErrorCode.TOO_MANY_LOGIN_ATTEMPTS));
+
+        verify(authenticationManager, never()).authenticate(any());
+    }
+
+    @Test
+    void login_fires_LOGIN_RATE_LIMITED_audit_when_rate_limited() {
+        UUID userId = UUID.randomUUID();
+        doThrow(DomainException.tooManyRequests(ErrorCode.TOO_MANY_LOGIN_ATTEMPTS, "rate limited"))
+            .when(loginRateLimiter).checkAndConsume(IP, "user@test.de");
+
+        assertThatThrownBy(() -> authService.login("user@test.de", "pass", IP, UA))
+            .isInstanceOf(DomainException.class);
+
+        verify(auditService).log(eq(AuditKind.LOGIN_RATE_LIMITED), isNull(), isNull(),
+            argThat(payload -> IP.equals(payload.get("ip")) && "user@test.de".equals(payload.get("email"))));
+    }
+
+    @Test
+    void login_invalidates_rate_limit_on_success() {
+        UUID userId = UUID.randomUUID();
+        AppUser user = AppUser.builder().id(userId).email("user@test.de").build();
+        Authentication auth = new UsernamePasswordAuthenticationToken("user@test.de", null, Set.of());
+        when(authenticationManager.authenticate(any())).thenReturn(auth);
+        when(userService.findByEmail("user@test.de")).thenReturn(user);
+
+        authService.login("user@test.de", "pass123", IP, UA);
+
+        verify(loginRateLimiter).invalidateOnSuccess(IP, "user@test.de");
+    }
+
     @SuppressWarnings("unchecked")
     @Test
     void revokeOtherSessions_preserves_current_and_deletes_N_minus_1() {

backend/src/test/java/org/raddatz/familienarchiv/auth/LoginRateLimiterTest.java

@@ -0,0 +1,67 @@
+package org.raddatz.familienarchiv.auth;
+
+import org.junit.jupiter.api.BeforeEach;
+import org.junit.jupiter.api.Test;
+import org.raddatz.familienarchiv.exception.DomainException;
+import org.raddatz.familienarchiv.exception.ErrorCode;
+
+import static org.assertj.core.api.Assertions.assertThatThrownBy;
+import static org.assertj.core.api.Assertions.assertThatNoException;
+
+class LoginRateLimiterTest {
+
+    private LoginRateLimiter rateLimiter;
+
+    @BeforeEach
+    void setUp() {
+        RateLimitProperties props = new RateLimitProperties();
+        props.setMaxAttemptsPerIpEmail(10);
+        props.setMaxAttemptsPerIp(20);
+        props.setWindowMinutes(15);
+        rateLimiter = new LoginRateLimiter(props);
+    }
+
+    @Test
+    void tenth_attempt_from_same_ip_email_succeeds() {
+        for (int i = 0; i < 10; i++) {
+            assertThatNoException().isThrownBy(
+                () -> rateLimiter.checkAndConsume("1.2.3.4", "user@example.com"));
+        }
+    }
+
+    @Test
+    void eleventh_attempt_from_same_ip_email_throws_TOO_MANY_LOGIN_ATTEMPTS() {
+        for (int i = 0; i < 10; i++) {
+            rateLimiter.checkAndConsume("1.2.3.4", "user@example.com");
+        }
+
+        assertThatThrownBy(() -> rateLimiter.checkAndConsume("1.2.3.4", "user@example.com"))
+            .isInstanceOf(DomainException.class)
+            .satisfies(ex -> org.assertj.core.api.Assertions.assertThat(((DomainException) ex).getCode())
+                .isEqualTo(ErrorCode.TOO_MANY_LOGIN_ATTEMPTS));
+    }
+
+    @Test
+    void success_after_10_failures_resets_ip_email_bucket() {
+        for (int i = 0; i < 10; i++) {
+            rateLimiter.checkAndConsume("1.2.3.4", "user@example.com");
+        }
+
+        rateLimiter.invalidateOnSuccess("1.2.3.4", "user@example.com");
+
+        assertThatNoException().isThrownBy(
+            () -> rateLimiter.checkAndConsume("1.2.3.4", "user@example.com"));
+    }
+
+    @Test
+    void twentyfirst_attempt_from_same_ip_across_different_emails_throws() {
+        for (int i = 0; i < 20; i++) {
+            rateLimiter.checkAndConsume("1.2.3.4", "user" + i + "@example.com");
+        }
+
+        assertThatThrownBy(() -> rateLimiter.checkAndConsume("1.2.3.4", "attacker@example.com"))
+            .isInstanceOf(DomainException.class)
+            .satisfies(ex -> org.assertj.core.api.Assertions.assertThat(((DomainException) ex).getCode())
+                .isEqualTo(ErrorCode.TOO_MANY_LOGIN_ATTEMPTS));
+    }
+}