feat(library): a missing file stays in the playlist, greyed and unplayable — #2527

Every browse, discover and mix query filters missing_since, so a track
whose file vanished disappears from the places Minstrel chooses music.
A playlist is different: the entry is there because the user put it
there, and silently dropping it rewrites their list behind their back.

So playlists keep the row and mark it instead. ListPlaylistTracks now
carries missing_since (still deliberately unfiltered), the service
layer surfaces it as PlaylistTrack.Unavailable, and the wire gains
"unavailable" on each entry.

A missing entry also loses its stream_url. Refusing to hand out a URL
that cannot serve is stronger than trusting every client to honour the
flag, and "stream_url": null is a shape the clients already model --
PlaylistWire.streamUrl is documented nullable for the track-removed
case -- so an older build degrades to "present but not playable" with
no change.

Nothing is deleted here and nothing should be: the row, its play
history, its likes and its taste contribution all survive a file going
missing, because the file may come back (and #2528 will adopt it if it
comes back renamed).

Also corrects two comments that had drifted into lying. delete.go still
claimed the file-gone case was NOT auto-reconciled and told admins to
delete rows by hand -- untrue since f6d1cf24, and that exact staleness
is what produced drift #572. It now says what DeleteTrackFile really is:
the destructive admin action, which CASCADEs play_events and likes, and
is emphatically not the missing-file path. watcher.go claimed the
safety-net scan "covers anything missed"; the walk only covers
additions, and it is reconcile that covers removals.

internal/db/dbq/playlists.sql.go

@@ -262,9 +262,10 @@ func (q *Queries) ListAllPlaylistTracksForCollage(ctx context.Context, arg ListA
 
 const listPlaylistTracks = `-- name: ListPlaylistTracks :many
 SELECT pt.playlist_id, pt.position, pt.track_id, pt.title, pt.artist_name, pt.album_title, pt.duration_sec, pt.added_at, pt.pick_kind,
-       t.id           AS live_track_id,
-       albums.id      AS album_id,
-       artists.id     AS artist_id
+       t.id            AS live_track_id,
+       t.missing_since AS missing_since,
+       albums.id       AS album_id,
+       artists.id      AS artist_id
 FROM playlist_tracks pt
 LEFT JOIN tracks  t       ON t.id       = pt.track_id
 LEFT JOIN albums           ON albums.id  = t.album_id
@@ -274,24 +275,31 @@ ORDER BY pt.position
 `
 
 type ListPlaylistTracksRow struct {
-	PlaylistID  pgtype.UUID
-	Position    int32
-	TrackID     pgtype.UUID
-	Title       string
-	ArtistName  string
-	AlbumTitle  string
-	DurationSec int32
-	AddedAt     pgtype.Timestamptz
-	PickKind    *string
-	LiveTrackID pgtype.UUID
-	AlbumID     pgtype.UUID
-	ArtistID    pgtype.UUID
+	PlaylistID   pgtype.UUID
+	Position     int32
+	TrackID      pgtype.UUID
+	Title        string
+	ArtistName   string
+	AlbumTitle   string
+	DurationSec  int32
+	AddedAt      pgtype.Timestamptz
+	PickKind     *string
+	LiveTrackID  pgtype.UUID
+	MissingSince pgtype.Timestamptz
+	AlbumID      pgtype.UUID
+	ArtistID     pgtype.UUID
 }
 
 // Joined to tracks for the live track id (the service layer derives the
 // stream URL from it); LEFT JOIN preserves the row when track_id is NULL
 // (track was removed from the library). The denormalized snapshot fields
 // on playlist_tracks remain authoritative for title/artist/album text.
+//
+// Deliberately NOT filtered on missing_since (#2527), unlike every browse /
+// discover / mix query. A playlist entry is something the user put here on
+// purpose, so a missing file stays in the list and renders as a dead row
+// rather than silently vanishing; missing_since rides along so the service
+// layer can mark it unplayable.
 func (q *Queries) ListPlaylistTracks(ctx context.Context, playlistID pgtype.UUID) ([]ListPlaylistTracksRow, error) {
 	rows, err := q.db.Query(ctx, listPlaylistTracks, playlistID)
 	if err != nil {
@@ -312,6 +320,7 @@ func (q *Queries) ListPlaylistTracks(ctx context.Context, playlistID pgtype.UUID
 			&i.AddedAt,
 			&i.PickKind,
 			&i.LiveTrackID,
+			&i.MissingSince,
 			&i.AlbumID,
 			&i.ArtistID,
 		); err != nil {