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/auth.py

@@ -11,7 +11,7 @@ from app import sysconfig
 from app.config import settings
 from app.db import get_db
 from app.models import Invite, User
-from app.security import decrypt
+from app.security import decrypt_optional
 
 PASSWORD_MIN_LEN = 10
 
@@ -122,9 +122,10 @@ def purge_user(db: Session, user: User, background: BackgroundTasks) -> None:
     user_id = user.id
     tok = user.token
     # Both are stored encrypted — decrypt for the revoke call (the access-token fallback was passing
-    # ciphertext, so a token row with no refresh token silently failed to revoke).
+    # ciphertext, so a token row with no refresh token silently failed to revoke). decrypt_optional so a
+    # TOKEN_ENCRYPTION_KEY mismatch can't 500 the purge — a best-effort revoke degrades to "skip". (B-core-2)
     google_token = (
-        (decrypt(tok.refresh_token_enc) or decrypt(tok.access_token)) if tok else None
+        (decrypt_optional(tok.refresh_token_enc) or decrypt_optional(tok.access_token)) if tok else None
     )
     # Raw delete so Postgres applies ON DELETE CASCADE / SET NULL on every dependent table.
     db.execute(delete(User).where(User.id == user_id))