fix: move persistent queue state writes off the event loop (#980)

PersistentQueue.put/delete wrote the whole queue inline: serialize, write
a temp file, fsync it, rename, then fsync the directory. All of that ran
synchronously inside async callers, so on a slow or contended filesystem
a single queue mutation stalled every other request for as long as the
two fsyncs took. Adds and completions are exactly when it fires, which
matches the reported "hiccups happen when something is pushing into the
queue".

put/delete are now coroutines. The payload is still serialized on the
event loop -- it is pure CPU and sub-millisecond -- and only the write
goes to a thread, so the writer never walks live DownloadInfo objects
while the loop mutates them. Each queue gets its own single-worker
executor rather than sharing the default one, because extract_info can
hold default-executor threads for minutes and would leave state writes
queued behind exactly when they are needed.

Awaiting the write makes interleaving possible where it was not before,
so a lock now covers the mutate-write-rollback section: the invariant
that in-memory state never diverges from what is on disk is unchanged,
including the rollback when a write fails. On shutdown the queues are
drained rather than cancelled, so a write in flight still lands.

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

app/tests/test_download_queue.py

@@ -330,7 +330,7 @@ async def test_retry_restores_playlist_output_context(dq_env):
         chapter_template="",
     )
     failed_info.status = "error"
-    dq.done.put(Download(None, None, None, None, "best", "any", {}, failed_info))
+    await dq.done.put(Download(None, None, None, None, "best", "any", {}, failed_info))
 
     def fake_extract(self, extracted_url, *_args, **_kwargs):
         return {
@@ -389,7 +389,7 @@ async def test_retry_keeps_playlist_context_through_url_indirection(dq_env):
     dq = DownloadQueue(dq_env, notifier)
     url = "https://example.com/watch?v=1"
     resolved = "https://example.com/resolved?v=1"
-    dq.done.put(Download(None, None, None, None, "best", "any", {}, _failed_playlist_item(url)))
+    await dq.done.put(Download(None, None, None, None, "best", "any", {}, _failed_playlist_item(url)))
 
     def fake_extract(self, extracted_url, *_args, **_kwargs):
         if extracted_url == url:
@@ -427,7 +427,7 @@ async def test_retry_reapplies_current_options_gates(dq_env):
         ytdl_options_presets=["Still There", "Removed Preset"],
         ytdl_options_overrides={"paths": {"home": "/etc"}},
     )
-    dq.done.put(Download(None, None, None, None, "best", "any", {}, info))
+    await dq.done.put(Download(None, None, None, None, "best", "any", {}, info))
 
     def fake_extract(self, extracted_url, *_args, **_kwargs):
         return {
@@ -457,7 +457,7 @@ async def test_retry_keeps_overrides_while_still_allowed(dq_env):
     dq = DownloadQueue(dq_env, notifier)
     url = "https://example.com/watch?v=1"
     info = _failed_playlist_item(url, ytdl_options_overrides={"writesubtitles": True})
-    dq.done.put(Download(None, None, None, None, "best", "any", {}, info))
+    await dq.done.put(Download(None, None, None, None, "best", "any", {}, info))
 
     def fake_extract(self, extracted_url, *_args, **_kwargs):
         return {
@@ -481,7 +481,7 @@ async def test_retry_carries_the_sponsorblock_flag(dq_env):
     notifier = AsyncMock()
     dq = DownloadQueue(dq_env, notifier)
     url = "https://example.com/watch?v=1"
-    dq.done.put(
+    await dq.done.put(
         Download(None, None, None, None, "best", "any", {}, _failed_playlist_item(url, sponsorblock=True))
     )
 
@@ -1441,9 +1441,9 @@ async def test_post_download_cleanup_clears_filename_on_error(dq_env):
     notifier = AsyncMock()
     dq = DownloadQueue(dq_env, notifier)
     download = _make_download(dq_env, status="downloading", filename="../tmp/partial.mp4")
-    dq.queue.put(download)
+    await dq.queue.put(download)
 
-    dq._post_download_cleanup(download)
+    await dq._post_download_cleanup(download)
 
     assert download.info.status == "error"
     assert download.info.filename is None
@@ -1456,9 +1456,9 @@ async def test_post_download_cleanup_keeps_captured_subtitles_on_error(dq_env):
     dq = DownloadQueue(dq_env, notifier)
     download = _make_download(dq_env, download_type="captions", status="downloading", filename="en.srt")
     download.info.subtitle_files = [{"filename": "en.srt", "size": 42}]
-    dq.queue.put(download)
+    await dq.queue.put(download)
 
-    dq._post_download_cleanup(download)
+    await dq._post_download_cleanup(download)
 
     assert download.info.status == "error"
     assert download.info.filename == "en.srt"
@@ -1478,7 +1478,7 @@ async def test_clear_skips_deletion_outside_download_directory(dq_env):
     # A crafted/legacy relative filename that escapes DOWNLOAD_DIR via '..'.
     escaping_filename = os.path.relpath(outside_file, dq_env.DOWNLOAD_DIR)
     download = _make_download(dq_env, status="finished", filename=escaping_filename)
-    dq.done.put(download)
+    await dq.done.put(download)
 
     await dq.clear([download.info.url])