Fix auth redirect on landing page and inconsistent auth state

[emares17]

Summary

• Authenticated users returning to / are now redirected to /dashboard instead of seeing the marketing landing page
• get_current_user() now validates token expiry and attempts a refresh before returning the user, making /auth/status consistent with what verify_session_auth enforces on upload/preview endpoints

Root causes

Bug 1: The / route rendered <LandingPage /> unconditionally — only /login had an auth check redirect. Returning users with a valid session saw the marketing page with the nav showing them as logged in, but had to navigate to dashboard manually.

Bug 2: get_current_user() (used by /auth/status) only checked user_id in session with no token validation. verify_session_auth (on /upload and /preview) checked token expiry and tried to refresh. If a session's access token expired (15 min) and the refresh token was also expired (7 days), the nav and ProtectedRoute showed the user as authenticated while uploads returned 401. Observed via PostHog: users reaching the upload page with no label_generation_completed events.

Changes

• backend/auth/decorators.py — get_current_user() now checks access_token_expires, calls refresh_access_token() if expired, and returns None if refresh fails
• frontend-vite/react-ts/src/pages/LandingPage.tsx — added auth check useEffect on mount, redirects to /dashboard if authenticated, shows spinner while checking (mirrors existing pattern in login.tsx)

Test plan

• Log in, navigate to / — should redirect to /dashboard
• Log out, navigate to / — should show landing page normally
• Simulate expired tokens (shorten TTLs temporarily) — navigating to a protected route after both tokens expire should redirect to /login, not let through and fail on upload
• All existing backend integration tests pass
• All existing Playwright e2e tests pass

Closes #18

[railway-app]

🚅 Deployed to the BarcodeGenerator-pr-19 environment in LabelGenius

Service	Status	Web	Updated (UTC)
LabelGenius-Frontend	✅ Success (View Logs)	Web	Apr 29, 2026 at 10:29 pm
LabelGenius-Backend	✅ Success (View Logs)	Web	Apr 29, 2026 at 10:18 pm