refactor: optimize database session lifecycle and middleware architecture

- Improve database pool capacity logging with detailed configuration parameters
- Optimize database session dependency injection with middleware-managed lifecycle
- Simplify plugin middleware by delegating session creation to FastAPI dependencies
- Fix import path in auth routes (relative to absolute)
- Add safety checks for database session management across middleware exception handlers
- Ensure session cleanup only when not managed by middleware (avoid premature cleanup)

src/api/auth/routes.py

@@ -211,7 +211,7 @@ class AuthRefreshAdapter(AuthPublicAdapter):
 
 class AuthRegisterAdapter(AuthPublicAdapter):
     async def handle(self, context):  # type: ignore[override]
-        from ..models.database import SystemConfig
+        from src.models.database import SystemConfig
 
         db = context.db
         payload = context.ensure_json_body()