feat: YouTube Music audio capture (ytm: shorthand, Spotify detection, stalled job recovery) (#19)

* feat: add YouTube Music and Spotify source detection

- Add Source variants: YouTubeMusicTrack, YouTubeMusicPlaylist,
  SpotifyTrack, SpotifyAlbum, SpotifyPlaylist
- ytm:ID shorthand → music.youtube.com/watch?v=ID (audio-only, forced
  in core regardless of caller quality hint)
- ytm:playlist/ID and music.youtube.com/playlist URLs detected but
  fail with 'not yet implemented' via fail_run
- Spotify URLs/shorthands detected and fail fast with clear DRM error
  via fail_run (after run item created, so status is visible in /runs)
- source_metadata: youtube_music/music/audio and spotify/music/audio
  (entity_kind='music' for UI pill, representation_kind='audio' stored)
- locator_to_ytdlp_url includes YouTubeMusicTrack for probe endpoint
- generate_entry_title: 'Title — Artist' for YTM tracks
- Frontend: isVideoSource handles ytm: and music.youtube.com/watch;
  Spotify returns false (no probe, clear server error on submit)
- Placeholder updated to include ytm:ID
- SOURCE_ICONS: youtube_music (red disc) and spotify (green waves)
- 14 new tests covering all new sources (163 total, all pass)

* fix: prevent yt-dlp playlist expansion and stalled run recovery

- Add --no-playlist to ytdlp::download and fetch_metadata: URLs with a
  list= parameter (e.g. music.youtube.com/watch?v=ID&list=RDAMVM…) no
  longer cause yt-dlp to expand the full playlist and hang; both the
  metadata probe and the download are now single-item only

- Fix fail_stalled_capture_jobs to also recover archive_runs and
  archive_run_items: capture_jobs.run_uid is NULL at crash time so a
  join is unreliable; instead fail all archive_runs/items still
  in_progress directly, then recount failed_count via subquery.
  Startup recovery now makes the Runs UI reflect the correct failed
  state after a hard shutdown

- Expand fail_stalled_jobs_on_restart test to assert archive_run and
  archive_run_item rows are also marked failed, not just capture_jobs

* fix: use play triangle for youtube_music icon

frontend/src/components/CaptureDialog.jsx

@@ -18,6 +18,11 @@ function isVideoSource(locator) {
     }
   }
 
+  // ytm: shorthand — track only; playlist is not yet implemented
+  if (ll.startsWith('ytm:')) {
+    return !ll.slice(4).startsWith('playlist/')
+  }
+
   // x: / twitter: / tweet: shorthands — only x:media:ID routes to yt-dlp (Source::X)
   for (const scheme of ['x:', 'twitter:', 'tweet:']) {
     if (ll.startsWith(scheme)) {
@@ -25,6 +30,9 @@ function isVideoSource(locator) {
     }
   }
 
+  // spotify: shorthands — all will fail with a clear error; no probe needed
+  if (ll.startsWith('spotify:')) return false
+
   // Other platform shorthands — all go to yt-dlp
   if (ll.startsWith('instagram:') || ll.startsWith('facebook:') ||
       ll.startsWith('tiktok:') || ll.startsWith('reddit:') ||
@@ -32,6 +40,8 @@ function isVideoSource(locator) {
 
   // HTTP/HTTPS URLs — match the same regexes and prefix checks as determine_source
   if (ll.startsWith('http://') || ll.startsWith('https://')) {
+    // YouTube Music track (watch) — before generic YouTube check
+    if (/^https?:\/\/music\.youtube\.com\/watch/.test(ll)) return true
     // YouTube video (watch, youtu.be, shorts) — not playlist or channel
     if (/^https?:\/\/(?:www\.)?(?:youtu\.be\/[0-9A-Za-z_-]+|youtube\.com\/watch\?v=[0-9A-Za-z_-]+|youtube\.com\/shorts\/[0-9A-Za-z_-]+)/.test(l)) return true
     // x.com → Source::X → yt-dlp (note: twitter.com URLs fall through to Source::Url, not yt-dlp)
@@ -46,6 +56,8 @@ function isVideoSource(locator) {
     if (/^https?:\/\/(?:www\.)?reddit\.com\//.test(ll) || ll.startsWith('https://redd.it/') || ll.startsWith('http://redd.it/')) return true
     // Snapchat
     if (/^https?:\/\/(?:www\.)?snapchat\.com\//.test(ll)) return true
+    // Spotify — all will fail with a clear error; no probe needed
+    if (ll.startsWith('https://open.spotify.com/') || ll.startsWith('http://open.spotify.com/')) return false
   }
 
   return false
@@ -402,7 +414,7 @@ function CaptureRow({ item, autoFocus, onLocatorChange, onQualityChange, onRemov
           ref={inputRef}
           className="capture-input"
           type="text"
-          placeholder="https://… · yt:ID · tweet:ID · x:ID"
+          placeholder="https://… · yt:ID · ytm:ID · tweet:ID · x:ID"
           value={item.locator}
           onChange={e => onLocatorChange(e.target.value)}
           onKeyDown={e => { if (e.key === 'Enter') onSubmit() }}