Phase 4: Circuit Breaker, DB Retry, Redis Graceful Degradation

- app/core/resilience.py: CircuitBreaker (CLOSED/OPEN/HALF_OPEN), retry_db,
  redis_call_with_fallback, InMemoryRateLimiter, CircuitBreakerMiddleware
- app/core/auth.py: get_session_data now falls back to PostgreSQL sessions
  table when Redis is unavailable
- app/core/permissions.py: get_cached_permissions falls back to direct DB
  resolution when Redis circuit is open
- app/core/rate_limit.py: check_rate_limit falls back to in-memory limiter
  when Redis is down; reset_rate_limit clears both Redis and in-memory
- app/core/middleware.py: CSRF validation uses get_session_data (Redis+DB
  fallback); sliding session TTL is best-effort during outage
- app/core/db/__init__.py: get_db() wraps session creation with retry_db
  for transient connection errors; records circuit breaker success/failure
- app/deps.py: refresh_session_ttl wrapped in try/except for Redis outage
- app/main.py: CircuitBreakerMiddleware registered (returns 503 when DB
  circuit is OPEN, skips health/metrics endpoints)
- app/config.py: Added resilience settings (thresholds, cooldown, retries)
- tests/test_resilience.py: 30 tests covering all patterns

30/30 resilience tests pass. No regressions in plugin lifecycle tests.

app/config.py

@@ -66,6 +66,13 @@ class Settings(BaseSettings):
     # Trusted proxy CIDRs (comma-separated) — only these proxies can set X-Forwarded-For
     trusted_proxy_cidrs: str = ""
 
+    # Resilience
+    circuit_breaker_failure_threshold: int = 5
+    circuit_breaker_window_seconds: int = 30
+    circuit_breaker_cooldown_seconds: int = 60
+    db_retry_max_attempts: int = 3
+    db_retry_base_delay: float = 0.1
+
     # Rate Limiting
     rate_limit_login_max: int = 5
     rate_limit_login_window: int = 900  # 15 min