fix(config): degrade gracefully on a TOKEN_ENCRYPTION_KEY mismatch instead of a 500 (B-core-2)

A rotated or missing TOKEN_ENCRYPTION_KEY makes every stored secret undecryptable.
sysconfig._raw_override called security.decrypt() with no guard, and it sits on
hot paths (/api/me via google_enabled, login, every YouTube scheduler job,
account purge) — so a key mismatch raised InvalidToken and 500'd the whole app,
the SPA wouldn't even boot, with no documented recovery.

Catch the decrypt failure in _raw_override -> warn once per key and fall back to
the env/config default. The YouTube client's refresh-token decrypt now raises a
clear YouTubeError ("reconnect") that every caller already handles, and
purge_user uses decrypt_optional so a best-effort revoke can't 500 the delete.
Document that the key must never change once secrets are stored (recovery: clear
the affected system_config rows and re-enter).

backend/app/youtube/client.py

@@ -79,7 +79,12 @@ class YouTubeClient:
         cached = decrypt_optional(tok.access_token)
         if cached and tok.expiry and tok.expiry > now + timedelta(seconds=60):
             return cached
-        refresh = decrypt(tok.refresh_token_enc)
+        try:
+            refresh = decrypt(tok.refresh_token_enc)
+        except Exception as exc:  # noqa: BLE001 — Fernet InvalidToken (TOKEN_ENCRYPTION_KEY changed)
+            # A key mismatch must degrade like any other token problem (caller handles YouTubeError),
+            # not surface as a raw 500 on every scheduler job / connect check. (B-core-2)
+            raise YouTubeError("Stored OAuth token is unreadable — reconnect your account") from exc
         if not refresh:
             raise YouTubeError("No refresh token; user must re-authenticate")
         resp = self._send(