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/tests/test_plex_image_sizes.py

@@ -48,7 +48,7 @@ def _setup(db, make_user, monkeypatch, tmp_path) -> TestClient:
     )
     db.commit()
     monkeypatch.setattr(app_state, "_configured_cache", True)
-    monkeypatch.setattr(playback, "resolved_user_id", lambda request: (user.id, True))
+    monkeypatch.setattr(playback, "session_user_id", lambda request: user.id)
     monkeypatch.setattr(playback, "PlexClient", _FakePlex)
     monkeypatch.setattr(imgcache, "CACHE_DIR", tmp_path)
     _FakePlex.calls = []