fix(r5): address the third /code-review high round — 7 findings

- the sweep guard anchors the rating_key to digits: the round-2 widening had
  degenerated to "anything ending in _<digits>" (backup_2024, release_10)
- RSS counts apply-side failures too, so a read-only database no longer reports
  a wholly failed poll as "ok — new=0, failed=0"
- the player reads/writes its volume directly (readAccount/writeAccount): the
  persisted-object hook rendered nothing and cost a second write per settle
- new tests: the sweep guard's accept/reject table, and run_ffmpeg's threading
  (drain, cancel while silent, stall watchdog, stderr tail on a nonzero exit)
- the breaker sentinel is checked one way (isinstance) at all four sites
- run_rss_poll is typed dict[str, int]

All three new test groups were mutation-checked: reverting each fix turns them
red (or, for the cancel/stall pair, hangs — which is the bug itself).

backend/app/sync/playlists.py

@@ -346,10 +346,11 @@ def push_playlist(db: Session, user: User, pl: Playlist, *, live=None) -> dict:
             created = 1
             # Brand-new playlist: just append every video in order.
             for vid in desired:
-                if breaker.run(
+                added = breaker.run(
                     lambda v=vid: yt.add_playlist_item(pl.yt_playlist_id, v),
                     lambda v=vid: failures.append(v),
-                ) is not FAILED:
+                )
+                if not isinstance(added, _Failed):
                     inserted += 1
             if not failures:
                 pl.synced_fingerprint = fingerprint(pl.name, desired)
@@ -382,10 +383,11 @@ def push_playlist(db: Session, user: User, pl: Playlist, *, live=None) -> dict:
         # Delete extras (local is authoritative).
         for it in current:
             if it["video_id"] not in desired_set:
-                if breaker.run(
+                dropped = breaker.run(
                     lambda i=it: yt.delete_playlist_item(i["item_id"]),
                     lambda i=it: failures.append(i["video_id"]),
-                ) is not FAILED:
+                )
+                if not isinstance(dropped, _Failed):
                     deleted += 1
         # Insert missing (append; the reorder pass below fixes positions).
         cur_set = set(cur_vids)
@@ -407,12 +409,13 @@ def push_playlist(db: Session, user: User, pl: Playlist, *, live=None) -> dict:
                 continue  # failed insert; skip
             if i < len(model) and model[i] == want:
                 continue
-            if breaker.run(
+            moved = breaker.run(
                 lambda w=want, pos=i: yt.move_playlist_item(
                     item_id_by_vid[w], pl.yt_playlist_id, w, pos
                 ),
                 lambda w=want: failures.append(w),
-            ) is FAILED:
+            )
+            if isinstance(moved, _Failed):
                 continue
             reordered += 1
             j = model.index(want)