feat(backend): implement graceful shutdown for container stop

Graceful shutdown ensures in-flight operations complete before process exits:
- Lifespan shutdown handler drains pending tasks with 25s timeout
- Scheduler stops accepting new jobs immediately
- HTTP session, external logging, scheduler lock, DB conn closed cleanly
- 25s Python timeout leaves 5s margin before Docker's 30s SIGKILL

Files changed:
- backend/app/main.py: enhanced _lifespan shutdown with task drain
- Docker/Dockerfile.backend: documented signal handling in header
- Docker/docker-compose.yml: added stop_grace_period: 30s
- Docker/compose.prod.yml: added stop_grace_period: 30s
- Docs/Deployment.md: new Graceful Shutdown section with sequence table
- Docs/TROUBLESHOOTING.md: new Graceful Shutdown Issues section

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

backend/app/routers/auth.py

@@ -31,7 +31,7 @@ from app.dependencies import (
     SettingsDep,
 )
 from app.exceptions import AuthenticationError, RateLimitError
-from app.models.auth import LoginRequest, LoginResponse, LogoutResponse
+from app.models.auth import LoginRequest, LoginResponse, LogoutResponse, SessionValidResponse
 from app.services import auth_service
 from app.utils.client_ip import get_client_ip
 from app.utils.constants import SESSION_COOKIE_NAME
@@ -114,11 +114,12 @@ async def login(
 
 @router.get(
     "/session",
+    response_model=SessionValidResponse,
     summary="Validate the current session",
 )
 async def validate_session(
     _: AuthDep,
-) -> dict[str, bool]:
+) -> SessionValidResponse:
     """Validate the current session.
 
     This endpoint requires a valid session and returns 200 if the session is
@@ -132,9 +133,9 @@ async def validate_session(
         _: The injected session object (unused, but its presence triggers validation).
 
     Returns:
-        A simple JSON object confirming the session is valid.
+        :class:`~app.models.auth.SessionValidResponse` confirming the session state.
     """
-    return {"valid": True}
+    return SessionValidResponse(valid=True)
 
 
 @router.post(