feat(invites): implement invite-based self-service registration backend

- V45 migration: invite_tokens + invite_token_group_ids tables
- InviteToken entity with @ElementCollection group IDs
- InviteService: code generation, validation, redemption (pessimistic lock prevents TOCTOU), revoke, list
- RateLimitInterceptor (Caffeine-backed, 10 req/min per IP) registered via WebMvcConfigurer
- AuthController: GET /api/auth/invite/{code} + POST /api/auth/register (both public)
- InviteController: GET/POST/DELETE /api/invites (ADMIN_USER permission)
- SecurityConfig: permitAll for new public auth endpoints
- ErrorCode: INVITE_NOT_FOUND, INVITE_EXHAUSTED, INVITE_REVOKED, INVITE_EXPIRED
- 36 new tests (InviteServiceTest, AuthControllerTest, InviteControllerTest)

Closes #269

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

backend/pom.xml

@@ -146,6 +146,12 @@
 			<artifactId>flyway-database-postgresql</artifactId>
 		</dependency>
 
+		<!-- Caffeine cache for in-memory rate limiting -->
+		<dependency>
+			<groupId>com.github.ben-manes.caffeine</groupId>
+			<artifactId>caffeine</artifactId>
+		</dependency>
+
 		<!-- OpenAPI / Swagger UI — enabled only in the dev Spring profile -->
 		<dependency>
 			<groupId>org.springdoc</groupId>