Never serve a partial file: it makes Jellyfin transcode

The 12s first-byte grace was the wrong trade and a real play found it
within the hour. A 2-hour upload took 3 minutes to start, played 6
seconds, and stalled. Jellyfin had run ffmpeg with -probesize 1G against
the growing stream and then transcoded to HLS with libx264.

The cause is the container. A fragmented MP4 with empty_moov has no
duration in its header, so the only way to get one is to sum every
fragment -- probing a growing file reads all of it. Jellyfin cannot
establish duration, codec or bitrate, so it abandons direct play and
transcodes a stream it also cannot seek. It was targeting 4.83 Mbps
against a source measured at 3.29: re-encoding a stream that already fit,
because it could not measure it.

The same video once complete reports SupportsDirectPlay with the exact
runtime and bitrate.

So FIRST_BYTE_GRACE defaults to infinite again, with --wait-timeout raised
to 600s for a 2-hour upload. A cold long video is slow to start, which is
accepted: the fetch outlives the request so a retry is instant, and a
retryable stall beats a transcode that wastes a gigabyte and cannot work.
Both failure modes are recorded at the constant in the order measured so
the 12s cap is not reintroduced.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

tests/test_runner.py

@@ -230,3 +230,37 @@ def test_summarise_is_a_single_line():
     assert "channels=119" in text
     assert "materialised=7" in text
     assert "aged_out=8" in text
+
+
+def test_run_prunes_channels_that_have_nothing_to_show(conn, settings, media_root,
+                                                       monkeypatch):
+    """End to end: a subscribed channel whose directory exists only because
+    subscribe() wrote tvshow.nfo must not survive a run as an empty series."""
+    from conftest import add_channel
+    from ytstream import runner
+
+    empty = add_channel(conn, "UC" + "s" * 22, "Dead Channel", "dead-channel")
+    tree = media_root / "dead-channel"
+    tree.mkdir()
+    (tree / "tvshow.nfo").write_text("<tvshow/>")
+
+    assert runner.prune_empty_channels(conn) == 1
+    assert not tree.exists()
+
+
+def test_run_keeps_a_channel_with_a_materialised_video(conn, settings, media_root):
+    from conftest import add_channel, add_video
+    from ytstream import runner, videos
+
+    row = add_channel(conn, "UC" + "t" * 22, "Alive", "alive")
+    tree = media_root / "alive"
+    (tree / "Season 2026").mkdir(parents=True)
+    (tree / "Season 2026" / "ep.strm").write_text("url")
+    video = add_video(conn, row["id"], "vid00000009")
+    videos.mark_materialised(conn, "vid00000009",
+                             rel_path="alive/Season 2026/ep.strm", season=2026,
+                             episode=1, upload_date="2026-08-01", duration=900,
+                             title="t")
+
+    assert runner.prune_empty_channels(conn) == 0
+    assert tree.exists()