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/config.py

@@ -190,6 +190,26 @@ class Settings(BaseSettings):
     admin_username: Optional[str] = None
     admin_password: Optional[str] = None
     session_secret: Optional[str] = None
+    session_lifetime_days: int = Field(
+        default=30,
+        description=(
+            "Session lifetime in days.  Common values: 30, 60, 90.  "
+            "Determines how long a user stays logged in before being required to re-authenticate.  "
+            "Applies to both browser sessions and the session cookie max_age."
+        ),
+    )
+    session_lifetime_custom_days: int | None = Field(
+        default=None,
+        description=(
+            "Override session_lifetime_days with a custom value.  "
+            "When set, this takes precedence over session_lifetime_days.  "
+            "Useful for admin-configured non-standard durations."
+        ),
+    )
+    qr_login_challenge_ttl_seconds: int = Field(
+        default=120,
+        description="Time-to-live in seconds for QR login challenges (default: 2 minutes).",
+    )
     admin_group_name: str = "admin"
 
     # Multi-user settings