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/admin.py

@@ -149,7 +149,7 @@ def watch_link_set(
     result: dict = {}
     if enabled and not link.initial_import_done:
         result = plex_watch.import_owner_watch_state(db, link)
-    return {**_link_dict(link), "import": result or None}
+    return {"ok": True, **_link_dict(link), "import": result or None}
 
 
 @router.post("/watch/import")
@@ -160,4 +160,4 @@ def watch_import_now(user: User = Depends(admin_user), db: Session = Depends(get
     if link is None or not link.sync_enabled:
         raise refused(409, "watch_sync_off")
     result = plex_watch.import_owner_watch_state(db, link)
-    return {**_link_dict(link), "import": result}
+    return {"ok": True, **_link_dict(link), "import": result}