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/feed.py

@@ -261,7 +261,7 @@ def set_video_state(
             # or moved this row. The state the caller asked for is the state that holds either way.
             except StaleDataError:
                 db.rollback()
-        return {"video_id": video_id, "status": "new"}
+        return {"ok": True, "video_id": video_id, "status": "new"}
 
     # watched / hidden: atomic upsert. The player marks "watched" at end-of-playback at
     # the same instant a progress checkpoint may DELETE the in-progress row; a plain
@@ -282,7 +282,7 @@ def set_video_state(
     )
     db.execute(stmt)
     db.commit()
-    return {"video_id": video_id, "status": status}
+    return {"ok": True, "video_id": video_id, "status": status}
 
 
 @router.delete("/videos/{video_id}/state")
@@ -304,7 +304,7 @@ def clear_video_state(
         # silent-ok: the row went under the delete, which is the outcome the delete wanted.
         except StaleDataError:
             db.rollback()
-    return {"video_id": video_id, "status": "new", "position_seconds": 0}
+    return {"ok": True, "video_id": video_id, "status": "new", "position_seconds": 0}
 
 
 class VideoProgressIn(BaseModel):
@@ -362,7 +362,7 @@ def set_video_progress(
             # silent-ok: exactly the race the comment above describes; the response stays true.
             except StaleDataError:
                 db.rollback()
-        return {"video_id": video_id, "position_seconds": 0}
+        return {"ok": True, "video_id": video_id, "position_seconds": 0}
 
     if row is None:
         row = VideoState(user_id=user.id, video_id=video_id)
@@ -370,7 +370,7 @@ def set_video_progress(
     row.position_seconds = position
     row.progress_updated_at = datetime.now(timezone.utc)
     db.commit()
-    return {"video_id": video_id, "position_seconds": position}
+    return {"ok": True, "video_id": video_id, "position_seconds": position}
 
 
 @router.get("/videos/{video_id}")