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/plex/playlists.py

@@ -325,7 +325,7 @@ def playlist_add_bulk(
         )
         or 0
     )
-    return {"added": added, "item_count": total}
+    return {"ok": True, "added": added, "item_count": total}
 
 
 @router.post("/playlists/{pid}/items/remove-bulk")
@@ -356,7 +356,7 @@ def playlist_remove_bulk(
         )
         or 0
     )
-    return {"removed": removed, "item_count": total}
+    return {"ok": True, "removed": removed, "item_count": total}
 
 
 @router.put("/playlists/{pid}/order")