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/runner.py

@@ -48,11 +48,15 @@ def get_service_user(db: Session) -> User | None:
     return users[0] if users else None
 
 
-def run_rss_poll(db: Session, channels: list[Channel] | None = None) -> dict:
-    """Poll every channel's free RSS feed. Returns `{"new": inserted, "failed": unreadable_feeds}`
-    — the failure count is part of the RESULT, not just a log line, so the scheduler card and the
-    audit summary can tell "no new uploads" apart from "every feed was unreachable" (an egress
-    outage otherwise renders identically to a quiet hour)."""
+def run_rss_poll(db: Session, channels: list[Channel] | None = None) -> dict[str, int]:
+    """Poll every channel's free RSS feed. Returns `{"new": inserted, "failed": channels_that_did
+    _not_complete}` — the failure count is part of the RESULT, not just a log line, so the scheduler
+    card and the audit summary can tell "no new uploads" apart from "nothing worked" (an egress
+    outage, or a database that can't accept writes, otherwise renders identically to a quiet hour).
+
+    `failed` counts a channel whose feed could not be READ *or* whose rows could not be APPLIED —
+    both leave the channel un-polled, and counting only the first would move the blind spot rather
+    than remove it."""
     if channels is None:
         channels = db.execute(select(Channel)).scalars().all()
     total = len(channels)
@@ -87,12 +91,15 @@ def run_rss_poll(db: Session, channels: list[Channel] | None = None) -> dict:
             try:
                 new += apply_rss_feed(db, channel, entries)
             except Exception:
+                # A write failure (read-only DB, failover) leaves the channel just as un-polled as
+                # an unreachable feed — count it, or a wholly broken run still reports "failed=0".
                 db.rollback()
+                failed += 1
                 log.exception("RSS apply failed for channel %s", channel.id)
             done += 1
             progress.report(done, total, "rss_poll")
     if failed:
-        log.warning("RSS poll: %d/%d channel feeds could not be read", failed, total)
+        log.warning("RSS poll: %d/%d channels could not be polled", failed, total)
     return {"new": new, "failed": failed}