feat(auth): add server-side session management and QR code login backend

- Add UserSession and QRLoginChallenge models for session tracking
  and mobile QR authentication
- Add session_manager utility with create/validate/revoke/cleanup
  functions and QR challenge helpers
- Add /api/sessions endpoints for listing, revoking, and
  'log off everywhere' functionality
- Add /api/qr-auth endpoints for challenge creation, polling, and
  claiming with API token issuance
- Add session config fields (lifetime, custom override, QR TTL)
- Update get_current_user to validate server-side sessions
- Create server-side sessions on all login paths (local, OAuth,
  social, admin)
- Revoke server-side session on logout
- Configure SessionMiddleware max_age from session lifetime settings
- Graceful degradation: old sessions without _session_token continue
  to work

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

app/main.py

@@ -318,8 +318,19 @@ app.add_middleware(CSRFMiddleware, config=settings)
 #    See SECURITY_AUDIT.md – Infrastructure Security section
 app.add_middleware(AuditLogMiddleware, config=settings)
 
+
 # 3) Session Middleware (for request.session to work)
-app.add_middleware(SessionMiddleware, secret_key=SESSION_SECRET)
+def _get_session_max_age() -> int:
+    """Compute session max-age at startup time."""
+    try:
+        from app.utils.session_manager import get_session_max_age_seconds
+
+        return get_session_max_age_seconds()
+    except Exception:
+        return 30 * 86400  # 30 days default fallback
+
+
+app.add_middleware(SessionMiddleware, secret_key=SESSION_SECRET, max_age=_get_session_max_age())
 
 # 3a) CORS Middleware - handles cross-origin requests and preflight (OPTIONS) responses.
 #     Disabled by default: set CORS_ENABLED=True only when NOT using a reverse proxy