fix(auth): sequential rate-limit check with ipEmail token refund on IP failure

Addresses Felix (blocker 1): the old implementation consumed from both buckets
before checking either result, silently eroding the per-email quota when only the
per-IP limit was blocking. The fix checks ipEmail first, then IP; on IP failure it
refunds the ipEmail token so legitimate users behind a shared IP are not penalised.

Also adds two new test cases:
- different_email_from_same_ip_not_blocked_by_sibling_email_exhaustion (Sara)
- ip_exhaustion_does_not_consume_ipEmail_tokens_for_blocked_attempts (red → green)

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

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

@@ -36,10 +36,18 @@ public class LoginRateLimiter {
                 .build(key -> newBucket(maxPerIp, windowMinutes));
     }
 
+    // NOTE: This cache is node-local (in-memory). In a multi-replica deployment,
+    // effective limits would be multiplied by replica count.
+    // For the current single-VPS setup this is the correct, simplest implementation.
+
     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) {
+        if (!byIpEmail.get(ip + ":" + email).tryConsume(1)) {
+            throw DomainException.tooManyRequests(ErrorCode.TOO_MANY_LOGIN_ATTEMPTS,
+                    "Too many login attempts from " + ip);
+        }
+        if (!byIp.get(ip).tryConsume(1)) {
+            // Refund the ipEmail token so IP-level blocking does not erode the per-email quota.
+            byIpEmail.get(ip + ":" + email).addTokens(1);
             throw DomainException.tooManyRequests(ErrorCode.TOO_MANY_LOGIN_ATTEMPTS,
                     "Too many login attempts from " + ip);
         }

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

@@ -64,4 +64,48 @@ class LoginRateLimiterTest {
             .satisfies(ex -> org.assertj.core.api.Assertions.assertThat(((DomainException) ex).getCode())
                 .isEqualTo(ErrorCode.TOO_MANY_LOGIN_ATTEMPTS));
     }
+
+    @Test
+    void different_email_from_same_ip_not_blocked_by_sibling_email_exhaustion() {
+        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);
+
+        assertThatNoException().isThrownBy(
+            () -> rateLimiter.checkAndConsume("1.2.3.4", "other@example.com"));
+    }
+
+    @Test
+    void ip_exhaustion_does_not_consume_ipEmail_tokens_for_blocked_attempts() {
+        // Use a tighter limiter so the phantom-consumption effect is observable.
+        // ipEmail=3, IP=3: exhausting IP via one email burns the other email's quota with the old code.
+        RateLimitProperties props = new RateLimitProperties();
+        props.setMaxAttemptsPerIpEmail(3);
+        props.setMaxAttemptsPerIp(3);
+        props.setWindowMinutes(15);
+        LoginRateLimiter tightLimiter = new LoginRateLimiter(props);
+
+        // Exhaust the per-IP bucket using "user@"
+        for (int i = 0; i < 3; i++) {
+            tightLimiter.checkAndConsume("1.2.3.4", "user@example.com");
+        }
+
+        // Three blocked attempts for "target@" while IP is exhausted
+        for (int i = 0; i < 3; i++) {
+            assertThatThrownBy(() -> tightLimiter.checkAndConsume("1.2.3.4", "target@example.com"))
+                .isInstanceOf(DomainException.class);
+        }
+
+        // A successful login for "user@" resets the IP bucket but NOT target@'s ipEmail bucket
+        tightLimiter.invalidateOnSuccess("1.2.3.4", "user@example.com");
+
+        // After IP reset: "target@" must NOT be blocked by an exhausted ipEmail bucket.
+        // With the old code, 3 blocked attempts burned all 3 ipEmail tokens → blocked here.
+        // With the fix, tokens are refunded on each blocked attempt → still has capacity.
+        assertThatNoException().isThrownBy(
+            () -> tightLimiter.checkAndConsume("1.2.3.4", "target@example.com"));
+    }
 }