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/sync.py

@@ -143,7 +143,7 @@ def deep_all(
         update(Subscription).where(Subscription.user_id == user.id).values(deep_requested=on)
     )
     db.commit()
-    return {"updated": result.rowcount, "deep_requested": on}
+    return {"ok": True, "updated": result.rowcount, "deep_requested": on}
 
 
 @router.post("/pause")
@@ -151,7 +151,7 @@ def pause_sync(admin: User = Depends(admin_user), db: Session = Depends(get_db))
     state.set_sync_paused(db, True)
     audit.record(db, admin.id, AuditAction.SYNC_PAUSE, summary="Background sync paused")
     db.commit()
-    return {"paused": True}
+    return {"ok": True, "paused": True}
 
 
 @router.post("/resume")
@@ -159,4 +159,4 @@ def resume_sync(admin: User = Depends(admin_user), db: Session = Depends(get_db)
     state.set_sync_paused(db, False)
     audit.record(db, admin.id, AuditAction.SYNC_RESUME, summary="Background sync resumed")
     db.commit()
-    return {"paused": False}
+    return {"ok": True, "paused": False}