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/tests/test_plex_stream.py

@@ -0,0 +1,44 @@
+"""The guard in front of a recursive delete.
+
+`sweep_orphans` rmtree's directories under `_HLS_ROOT`, and that root is admin-configurable
+(PLEX_HLS_DIR) — it can legitimately be a shared scratch path. `_SESSION_DIR_RE` is the ONLY thing
+that keeps the sweep off a neighbour's data, so the names it accepts are pinned here."""
+import pytest
+
+from app.plex.stream import _SESSION_DIR_RE
+
+
+@pytest.mark.parametrize(
+    "name",
+    [
+        "12345_0",  # legacy shape (pre multi-audio): {rating_key}_{start}
+        "12345_842",
+        "12345_0_None_+0.00",  # current shape, single audio track (audio_ord None)
+        "12345_0_2_+0.00",  # current shape, explicit audio_ord
+        "12345_842_multi_-1.25",  # multi-audio, negative offset
+        "12345_842_multi_+12.50",
+    ],
+)
+def test_our_session_directories_are_swept(name):
+    assert _SESSION_DIR_RE.match(name)
+
+
+@pytest.mark.parametrize(
+    "name",
+    [
+        "backup_2024",  # the class an "optional tail" pattern wrongly matched
+        "release_10",
+        "important_data_2024",
+        "node_modules",
+        "logs",
+        "plex-hls",
+        "12345",  # no start offset — not a session dir
+        "12345_0_multi",  # truncated: missing the offset
+        "12345_0_multi_1.25",  # offset without its sign
+        "snapshot_2024_backup_+1.00",  # session-ish tail, but the key isn't a rating_key
+        "backup_2024_multi_+0.00",
+        ".config",
+    ],
+)
+def test_foreign_directories_are_left_alone(name):
+    assert not _SESSION_DIR_RE.match(name)