refactor(auth): one account per browser session

The session held a wallet of up to six accounts, and a request picked one by a header or an
?account= parameter. It now holds one: user_id and its epoch. A sign-in replaces whoever was
signed in, sign-out and account deletion end the session, and an account refused mid-session
(revoked, suspended, deactivated) clears it. /api/me/accounts and /api/me/switch are gone, and
with them their refusal codes and the UI copy for them.

A cookie the multi-account build wrote carries no single epoch and is refused: its holder signs
in once more. A Google link finishing in a browser now signed in as someone else is refused, as
before for an account outside the wallet. A test holds that no app code reads the wallet keys or
the account header again.

backend/app/routes/plex/playback.py

@@ -19,7 +19,7 @@ from fastapi.responses import FileResponse
 from pydantic import BaseModel
 from sqlalchemy.orm import Session
 
-from app.auth import current_user, resolved_user_id
+from app.auth import current_user, session_user_id
 from app.db import SessionLocal, get_db
 from app.models import User
 from app.plex import imgcache, queries, service, subtitles
@@ -42,7 +42,7 @@ def _require_session(request: Request) -> int:
     """Lightweight auth for the high-fan-out image proxies: validate the signed session cookie WITHOUT
     a DB user-load, so these endpoints never hold a pool connection for the auth. Poster images are
     low-sensitivity; a valid session is enough."""
-    uid, _ = resolved_user_id(request)
+    uid = session_user_id(request)
     if not uid:
         raise refused(401, "not_authenticated")
     return uid