fix(auth): /api/me answers 200 when logged out (no console error)

The logged-out landing probed /api/me, which 401'd, and the browser logs every
non-2xx fetch as a console error regardless of how the app handles it. The
bootstrap probe now returns 200 with {authenticated:false} via a new
optional_current_user dependency; api.me() maps that to null. The render gate
treats no-data as 'signed out' -> the landing, and a thrown error as a real
network/5xx failure. Other protected endpoints still 401, so mid-session expiry
is still caught by the global handler. Lighthouse (dev): Best Practices 96->100.

backend/app/auth.py

@@ -719,6 +719,18 @@ def current_user(request: Request, db: Session = Depends(get_db)) -> User:
     return user
 
 
+def optional_current_user(
+    request: Request, db: Session = Depends(get_db)
+) -> User | None:
+    """Like `current_user` but returns None instead of raising 401 when there's no valid session.
+    Used by the bootstrap /api/me probe so the logged-out landing doesn't log a failed request to
+    the browser console (a non-2xx fetch is a console error no matter how the app handles it)."""
+    try:
+        return current_user(request, db)
+    except HTTPException:
+        return None
+
+
 def require_human(user: User = Depends(current_user)) -> User:
     """Reject the shared demo account from actions that need a real Google/YouTube identity
     or spend the shared quota. Most YouTube paths are already closed to it implicitly (it has