fix(r5): address the /code-review high round — 10 findings

- HLS sweep only deletes directories matching the session-name shape; the root
  is admin-configurable and may be a shared path
- playlist unwrap picks the first entry that actually downloaded, not entries[0]
- concat quoting keeps backslashes literal (ffmpeg treats them so inside '')
- ArrowUp/Down defer while the focused element can still scroll that way
- the staging fallback skips yt-dlp working files (.part-Frag/.ytdl/.temp/.fNNN)
- the boot HLS sweep runs off the event loop
- volume persistence is debounced (was a setItem per wheel notch) and a spin no
  longer loses notches to the iframe's lagging getVolume
- _Breaker.run owns the whole open/call/record protocol; four call sites shrink
- should_prune extracted and unit-tested, plus RSS error-vs-empty tests
- the worker exits non-zero when the schema never lands

backend/app/sync/subscriptions.py

@@ -73,6 +73,18 @@ def apply_channel_details(db: Session, items: list[dict]) -> None:
         channel.details_synced_at = now
 
 
+def should_prune(fetched_count: int, existing_count: int) -> bool:
+    """Whether an import may delete the local subscriptions missing from this fetch.
+
+    NEVER on an empty fetch that contradicts local state: the API can answer 200 with no items (a
+    transient blip, a scope/token edge), and pruning against that wipes every subscription the user
+    has — taking each row's `deep_requested` flag with it — in a single run. Someone who genuinely
+    unsubscribed from everything simply keeps their rows until a sync that returns something.
+
+    Pure so it can be tested without a database; `import_subscriptions` is the only caller."""
+    return fetched_count > 0 or existing_count == 0
+
+
 def import_subscriptions(db: Session, user: User) -> dict:
     """Pull all subscriptions for `user`, upsert channels + subscription links, prune
     channels the user has unsubscribed from on YouTube, and fetch channel details for
@@ -108,18 +120,15 @@ def import_subscriptions(db: Session, user: User) -> dict:
             sub_row.yt_subscription_id = sub.get("yt_subscription_id")
         db.flush()
 
-        # Prune subscriptions removed on YouTube — but NEVER on an empty fetch. The API can answer
-        # 200 with no items (a transient blip, a scope/token edge), and pruning against that wipes
-        # every subscription the user has, taking each row's `deep_requested` flag with it, in one
-        # run. Someone who genuinely unsubscribed from everything simply keeps their rows until a
-        # sync that returns something.
+        # Prune subscriptions removed on YouTube — but only when this fetch is trustworthy
+        # (see `should_prune`).
         removed = 0
         existing = (
             db.execute(select(Subscription).where(Subscription.user_id == user.id))
             .scalars()
             .all()
         )
-        prune_skipped = bool(existing) and not fetched
+        prune_skipped = not should_prune(len(fetched), len(existing))
         if prune_skipped:
             log.warning(
                 "Subscription import (user %s): YouTube returned 0 subscriptions while %d exist "