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/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