feat(auth): add server-backed demo sessions

The browser treated sessionStorage as the source of truth for the logged-in
user and never verified or invalidated the server-side session cookie: no
GET /api/v1/demo/session or POST /api/v1/demo/logout endpoint existed, and a
central 401 handler was defined but never wired up.

Add both endpoints; the session-check response is marked Cache-Control:
no-store to avoid the browser serving a stale "authenticated" response right
after logout. AuthProvider now verifies against the server on every mount
(sessionStorage only caches presentation state to avoid a login-screen
flash), subscribes to a central 401 listener on the API client, and
RequireAuth shows a loading state during verification instead of flashing
protected content or the wrong role.

backend/app/api/routers/demo.py

@@ -1,14 +1,15 @@
 from __future__ import annotations
 
 import time
+import uuid
 
-from fastapi import APIRouter, Depends, Response
+from fastapi import APIRouter, Depends, Request, Response
 from sqlalchemy import select
 from sqlalchemy.orm import Session
 
-from app.api.deps import get_db, require_operations_manager
+from app.api.deps import get_current_user, get_db, require_operations_manager
 from app.core.config import get_settings
-from app.core.security import SessionPayload, create_session_token
+from app.core.security import SessionPayload, create_session_token, read_session_token
 from app.models.user import User
 from app.schemas import CurrentUser, DemoLoginRequest
 from app.seed_loader import reset_and_seed
@@ -41,6 +42,7 @@ def demo_login(
         token,
         httponly=True,
         samesite="lax",
+        secure=settings.session_cookie_secure,
         max_age=settings.session_ttl_seconds,
     )
     record_audit_event(
@@ -56,6 +58,34 @@ def demo_login(
     return CurrentUser(public_ref=user.public_ref, display_name=user.display_name, role=body.role)
 
 
+@router.get("/session", response_model=CurrentUser)
+def get_session(
+    response: Response, user: CurrentUser = Depends(get_current_user)
+) -> CurrentUser:
+    # Never let the browser (or an intermediary) cache an authentication check — a stale
+    # cached 200 here would keep showing a logged-out browser as authenticated.
+    response.headers["Cache-Control"] = "no-store"
+    return user
+
+
+@router.post("/logout")
+def demo_logout(request: Request, response: Response, db: Session = Depends(get_db)) -> dict:
+    token = request.cookies.get(settings.session_cookie_name)
+    payload = read_session_token(token) if token else None
+    if payload is not None:
+        record_audit_event(
+            db,
+            actor_type="user",
+            actor_id=uuid.UUID(payload.user_id),
+            actor_label=payload.display_name,
+            action="demo_logout",
+            entity_type="user",
+        )
+        db.commit()
+    response.delete_cookie(settings.session_cookie_name)
+    return {"status": "logged_out"}
+
+
 @router.post("/reset")
 def demo_reset(
     response: Response,

backend/app/core/config.py

@@ -25,6 +25,7 @@ class Settings(BaseSettings):
     app_secret: str = "replace-in-production"
     session_cookie_name: str = "mobilityops_session"
     session_ttl_seconds: int = 60 * 60 * 8
+    session_cookie_secure: bool = False
     seed_dir: str = "/app/seed"
     knowledge_dir: str = "/app/knowledge/procedures"
     mcp_hub_service_token: str = "replace-me-mcp-hub-token"

backend/tests/test_auth.py

@@ -18,3 +18,32 @@ def test_operations_manager_can_reset_demo(ops_client):
     response = ops_client.post("/api/v1/demo/reset")
     assert response.status_code == 200
     assert response.json()["counts"]["vehicles"] == 50
+
+
+def test_session_endpoint_requires_authentication(client):
+    response = client.get("/api/v1/demo/session")
+    assert response.status_code == 401
+
+
+def test_session_endpoint_confirms_logged_in_user(ops_client):
+    response = ops_client.get("/api/v1/demo/session")
+    assert response.status_code == 200
+    body = response.json()
+    assert body["role"] == "operations_manager"
+    assert body["public_ref"] == "USR-OPS"
+
+
+def test_logout_invalidates_session(ops_client):
+    confirmed = ops_client.get("/api/v1/demo/session")
+    assert confirmed.status_code == 200
+
+    logout = ops_client.post("/api/v1/demo/logout")
+    assert logout.status_code == 200
+
+    after = ops_client.get("/api/v1/demo/session")
+    assert after.status_code == 401
+
+
+def test_logout_without_a_session_is_safe(client):
+    response = client.post("/api/v1/demo/logout")
+    assert response.status_code == 200