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/tests/test_plex_finish.py

@@ -76,5 +76,5 @@ def test_the_finish_supersedes_a_resume_push_still_in_flight(db, as_user, make_u
     r = as_user(user).post(
         "/api/plex/item/m1/progress", json={"position_seconds": 2995, "duration_seconds": 3000}
     )
-    assert r.json() == {"status": "watched", "position_seconds": 0}
+    assert r.json() == {"ok": True, "status": "watched", "position_seconds": 0}
     assert playback._resume_push_superseded(user.id, item.id, earlier) is True