fix(api): mutations answer ok; route helpers flush and leave the commit to the handler

Two rows of the canonical-patterns table had drifted, found while closing the roadmap audit:

- 63 mutation responses carried no `ok` — `{"unsubscribed": true}`, `{"status": "ok"}`,
  `{"added": 3, …}` and so on. Each now leads with `"ok": true` (additive: every field a client
  reads is still there).
- Twelve route helpers committed on their own. Six were just the end of their handler's work and now
  flush: the playlist item add/remove and Watch later get-or-create, the Plex series/season bulk
  mark, and the invite decision + account activation — which an approval used to land in separate
  transactions. The other six commit on purpose (a token durable before its link is mailed, a
  single-use token burned as its own unit, the Google callback's tail, a background registration
  with its own session, the staged search ingest around YouTube calls), each listed with its reason.

`tests/test_route_conventions.py` holds both rules in code; CLAUDE.md's two rows point at it.

backend/app/routes/setup.py

@@ -106,7 +106,7 @@ def setup_config(settings_map: dict[str, Any], db: Session = Depends(require_set
             raise setting_refused(exc) from None
         saved.append(key)
     db.commit()  # sysconfig.set_value only flushes now — persist the wizard's config in one commit
-    return {"saved": saved}
+    return {"ok": True, "saved": saved}
 
 
 class SetupTestEmailIn(BaseModel):