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

Five of them sit where round 6 turned F from a Fullscreen API call into a CSS
maximise: the guards around it still asked `isFullscreen`, which that pivot made
permanently false.

- Arrow-key volume was dead in the maximised view: the scroll-deference checked
  only `isFullscreenRef`, so the (now hidden but still scrollable) card kept
  swallowing Up/Down. Both flags now count as "the video fills the screen".
  Measured: 87px of scroll room on the focused card, ArrowDown moves 80 -> 75
  and scrolls nothing.
- The maximised view had no way out once the pointer touched the video. Reaching
  YouTube's controls means clicking into the cross-origin iframe, which takes
  keyboard focus with it, and the re-arm paths (stage mouseleave, window focus)
  need the pointer to leave the browser entirely while the stage fills it. So
  the exit now lives on screen, in our DOM: a button that un-maximises AND pulls
  focus back, re-arming every shortcut.
- WebKit fires only `webkitfullscreenchange`/`webkitFullscreenElement`, so on
  Safari/iPadOS YouTube's own fullscreen never registered — no overlay yield, no
  focus reclaim. One `fullscreenEl()` reader, both event spellings.
- The stage claimed `z-index: 9999`, the glass tuner's reserved level, for a job
  that only needs to beat the modal chrome it is nested in (its siblings top out
  at z-menu). Dropped to rail-level; nothing escapes the modal's stacking
  context either way.
- `inset: 0` already sizes a fixed layer — the inherited `100vw`/`100vh`
  over-constrained it, which is how a classic scrollbar gets a horizontal
  overflow and a mobile URL bar hides the bottom edge (YouTube's control bar).
- The shortcut hint still promised "F: fullscreen" in both locales.

Backend, both "the fix stopped one layer short":
- The startup HLS sweep was awaited inside lifespan, so uvicorn still served
  nothing until it finished — a thread doesn't help when nothing is listening
  yet. Fire-and-forget task instead, cancelled on shutdown. Measured with 200
  planted segments: "Application startup complete" now precedes "swept 1".
- An RSS poll where every channel failed returned normally, so the scheduler
  recorded `ok` with the outage buried in the summary string. `failed == total`
  now raises: a run that reached nothing is a failed run.

Suites: backend 147 -> 150.

backend/app/routes/sync.py

@@ -52,9 +52,10 @@ def sync_subscriptions(
 def sync_rss(
     user: User = Depends(require_human), db: Session = Depends(get_db)
 ) -> dict:
+    # A partial failure comes back in `failed_feeds` so a manual poll reports it instead of an
+    # innocent-looking "0 new"; a TOTAL outage raises out of run_rss_poll and surfaces as a 500,
+    # which is the honest answer for a poll that reached nothing.
     result = run_rss_poll(db, _user_channels(db, user))
-    # `failed` = feeds that could not be read at all (see run_rss_poll); surfaced so a manual poll
-    # reports an outage instead of an innocent-looking "0 new".
     return {"new_videos": result["new"], "failed_feeds": result["failed"]}