fix: harden download lifecycle, subscriptions, and UI robustness

Addresses a full-project review. Backend correctness and availability:

- ytdl: cancel() only SIGKILLs the child's process group when the child
  actually became its own group leader, so a race (or failed setpgrp)
  can no longer kill the whole server; kill the group on cancel and on
  shutdown to avoid orphaned ffmpeg children
- ytdl: dedicated ThreadPoolExecutor for download supervision so active
  downloads can't starve extract_info / live probes on the default pool
- ytdl/main/subscriptions: route fire-and-forget tasks through a
  bg_tasks helper that keeps a strong ref and logs failures
- subscriptions: run flat-playlist extraction in an executor and check
  feeds with bounded concurrency so one slow feed can't block the loop;
  set last_checked on failure so broken feeds aren't retried every 60s
- main: validate ids on /start & /delete and numeric env vars at startup;
  return 400 (not 500) on bad subscriptions/update input; serve /history
  from memory; move get_custom_dirs off the event loop; restrict t=
  stripping to YouTube hosts; drop double percent-decode in state guard
- dl_formats/ytdl: enforce requested caption format via
  FFmpegSubtitlesConvertor and strip VTT header metadata only in the
  pre-cue region so real dialogue is preserved
- ytdl: throttle progress events, dedup adds against pending, clear
  filename/size on error and reject out-of-dir trashcan deletes, pin
  fork start-method on Linux only

Frontend:

- retry deletes the done record only after a successful re-add
- surface HTTP errors for delete/start and reset the deleting flag
- ignore late 'updated' events for rows no longer in the queue
- track table rows by map key; FileSizePipe uses base-1024

Also: HTTPS-aware Docker healthcheck, dead-code removal, and shared
helpers for path-containment and yt-dlp option merging. Adds/updates
unit tests throughout (250 backend tests passing).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

app/tests/test_config.py

@@ -159,6 +159,35 @@ class ConfigTests(unittest.TestCase):
             c = Config()
         self.assertEqual(c.CLEAR_COMPLETED_AFTER, "0")
 
+    def test_invalid_default_option_playlist_item_limit_exits(self):
+        for bad in ("-1", "many"):
+            with patch.dict(os.environ, _base_env(DEFAULT_OPTION_PLAYLIST_ITEM_LIMIT=bad), clear=False):
+                with self.assertRaises(SystemExit):
+                    Config()
+
+    def test_default_option_playlist_item_limit_zero_allowed(self):
+        with patch.dict(os.environ, _base_env(DEFAULT_OPTION_PLAYLIST_ITEM_LIMIT="0"), clear=False):
+            c = Config()
+        self.assertEqual(c.DEFAULT_OPTION_PLAYLIST_ITEM_LIMIT, "0")
+
+    def test_invalid_subscription_default_check_interval_exits(self):
+        for bad in ("0", "-1", "often"):
+            with patch.dict(os.environ, _base_env(SUBSCRIPTION_DEFAULT_CHECK_INTERVAL=bad), clear=False):
+                with self.assertRaises(SystemExit):
+                    Config()
+
+    def test_invalid_subscription_scan_playlist_end_exits(self):
+        for bad in ("0", "-1", "all"):
+            with patch.dict(os.environ, _base_env(SUBSCRIPTION_SCAN_PLAYLIST_END=bad), clear=False):
+                with self.assertRaises(SystemExit):
+                    Config()
+
+    def test_invalid_subscription_max_seen_ids_exits(self):
+        for bad in ("0", "-1", "unlimited"):
+            with patch.dict(os.environ, _base_env(SUBSCRIPTION_MAX_SEEN_IDS=bad), clear=False):
+                with self.assertRaises(SystemExit):
+                    Config()
+
     def test_runtime_override_roundtrip(self):
         with patch.dict(os.environ, _base_env(), clear=False):
             c = Config()