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/core/middleware.py

@@ -111,18 +111,17 @@ class CSRFMiddleware(BaseHTTPMiddleware):
                     content={"detail": "No session for CSRF validation", "code": "csrf_no_session"},
                 )
 
-            # Look up CSRF token from Redis session (use singleton)
-            from app.core.auth import get_redis
+            # Look up CSRF token from session (Redis with DB fallback)
+            from app.core.auth import get_redis, get_session_data
 
             redis = get_redis()
-            raw = await redis.get(f"session:{session_id}")
-            if raw is None:
+            session_data = await get_session_data(redis, session_id)
+            if session_data is None:
                 return JSONResponse(
                     status_code=status.HTTP_403_FORBIDDEN,
                     content={"detail": "Session expired for CSRF validation", "code": "csrf_session_expired"},
                 )
 
-            session_data = json.loads(raw)
             stored_token = session_data.get("csrf_token")
 
             if not stored_token or stored_token != csrf_header:
@@ -132,6 +131,10 @@ class CSRFMiddleware(BaseHTTPMiddleware):
                 )
 
             # Sliding session: also extend TTL on CSRF-validated unsafe requests
-            await redis.expire(f"session:{session_id}", settings.session_ttl_seconds)
+            # (best-effort — ignore Redis errors during outage)
+            try:
+                await redis.expire(f"session:{session_id}", settings.session_ttl_seconds)
+            except Exception:
+                pass
 
         return await call_next(request)