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

The worst one was, again, the previous round's own fix.

- Round 7 stopped awaiting the HLS sweep so startup could serve immediately — which
  also made it run CONCURRENTLY with playback, and `sweep_orphans` snapshotted the
  live-session set BEFORE listing the root and rmtree'd outside the lock. Since
  `start_session` reuses a directory path for the same (key, offset, tag), a stale
  name can become a live session at any moment. Two guards now: nothing whose mtime
  is newer than this process's start is ever deleted, and the live check + rmtree
  happen together under the lock, one directory at a time.
- The sweep task is awaited after cancel, so shutdown can't leave it pending.
- `_rate_limiter` used a truthiness test on its timestamp, so a `now()` of exactly
  0.0 read as "never ran" — unreachable in production, but `now` exists to be
  injected and a test clock starting at 0 is the obvious choice.
- The RSS total-outage error now carries the first underlying error: "all N channels
  failed" is also what ONE dead feed id looks like on a small instance, and a red
  card the user can't act on is its own kind of dishonest.
- The WebKit fullscreen handling moved into `lib/fullscreen` and PlexPlayer uses it
  too — it had the same unprefixed-only reads in three places, including an Escape
  branch that would have closed the whole player instead of leaving fullscreen.
- The `--max` CSS comment claimed `inset: 0` did the sizing; Tailwind's `w-full` is
  still on the element and wins. Comment now states the real mechanism.

Verified in real Chrome (the pane can't composite or do native fullscreen):
- F fills the 1920x889 viewport; the exit pill renders top-right on the letterbox
  band and overlaps NO YouTube control — the collision this round suspected does not
  reproduce in this embed (its own buttons sit bottom-left / bottom-right).
- Four real ArrowDown presses while maximised: persisted volume 100 -> 80, with the
  level flash visible on screen.
- A real click on the exit pill un-maximises and returns focus to the card.
- lib/fullscreen driven from a real click: enter (innerHeight 889 -> 1024), the
  change listener fires exactly once per transition, exit returns to 889.

Backend suite 150 -> 158; the new sweep tests are mutation-checked, and they read
their cutoff from the filesystem rather than a clock (the container VM and the
bind-mounted host disagree by enough to make a clock-based assertion flaky).

backend/app/worker.py

@@ -330,11 +330,17 @@ _CANCEL_POLL_INTERVAL_S = 1.0
 def _rate_limiter(interval_s: float, *, now=time.monotonic):
     """`should_run()` → True at most once per `interval_s` (the first call always passes).
     `now` is injectable so tests drive a clock instead of patching the global one."""
-    state = {"at": 0.0}
+    # `None`, not 0.0: "never ran" has to be a VALUE, not a falsy timestamp. A truthiness test here
+    # reads a legitimate `now() == 0.0` as "never ran" and lets a second call straight through —
+    # unreachable with real `time.monotonic()`, but `now` exists to be injected, and a test clock
+    # starting at 0 is the obvious choice. That would let a throttle test encode the wrong
+    # behaviour and still pass.
+    state: dict[str, float | None] = {"at": None}
 
     def should_run() -> bool:
         t = now()
-        if state["at"] and t - state["at"] < interval_s:
+        last = state["at"]
+        if last is not None and t - last < interval_s:
             return False
         state["at"] = t
         return True