feat(library): detect missing files and stop offering them — #2523

Nothing in Minstrel ever noticed a deleted file. The walk only visits
paths that exist, so a row whose file was gone was never scanned, never
errored, never counted — permanently invisible. classifyEvent ignores
fsnotify removals by design, and the safety-net scan is the same walk, so
it covers additions only. Rows accumulated forever.

Found on the operator's library: a completed scan reported
skipped=24185 errored=0 while the MBID backfill (which opens files by DB
path rather than walking) logged ~40 "no such file or directory" across
three reorganised albums. Those rows also kept their pre-#2499 welded
genre, which is how this surfaced — the version-stamped tag re-read can
only reach files the walk visits.

The harm is not cosmetic. tracks is the candidate universe for
recommendation.sql / discover.sql / system_mixes.sql and nothing filtered
on file existence, so a mix could spend a slot on a track that cannot
stream.

Marks rather than deletes. A missing file is a claim about the filesystem
and the filesystem lies transiently — an unmounted volume, a network
blip, a container that started before its media mount attached. Every
sweep in internal/gc resolves a truth INSIDE the database and is safe to
run blind; this one is not, so no deletion happens here. Three guards
refuse to act on ambiguous evidence: every scan root must resolve to a
non-empty directory, the walk must have seen at least one file, and one
reconcile may newly mark at most 25% of the library. Clearing a mark is
never the dangerous direction, so it runs unconditionally — otherwise a
library that tripped the cap could never recover once the mount returned.

Only a full Scan reconciles. The walk's set of seen paths is the
evidence, and ScanFiles has no basis for concluding anything about files
it did not look at.

Excludes marked tracks from all 13 track-emitting queries (radio x2,
system mixes x5, discover x4, most-played x2), the 6 play-history seed
picks, and the genre browse axis. Deliberately NOT filtered: the shared
ListPlaylistTracks read path, because it also serves user-curated
playlists where hiding a track the user added would be wrong — system
playlists shed orphans on their next daily rebuild instead. History and
the taste profile also keep them: those record the past, and a track you
played 200 times still says something about your taste.

Reconcile tallies land in scan_runs so a disappearance is visible rather
than discovered when a mix comes up short.

internal/db/dbq/system_playlists.sql.go

@@ -189,6 +189,7 @@ func (q *Queries) GetSystemPlaylistRun(ctx context.Context, userID pgtype.UUID)
 
 const listActiveUsersForSystemPlaylists = `-- name: ListActiveUsersForSystemPlaylists :many
 
+
 SELECT u.id FROM users u
  WHERE EXISTS (
      SELECT 1 FROM play_events pe
@@ -197,6 +198,13 @@ SELECT u.id FROM users u
    )
 `
 
+// Track picks here join `tracks ... AND t.missing_since IS NULL` (#2523): a
+// seed or For-You candidate has to be something that can actually play. Note
+// this only affects newly GENERATED playlists — already-stored system
+// playlists keep their rows until the next daily rebuild, which is why the
+// shared ListPlaylistTracks read path is deliberately left unfiltered (it
+// also serves user-curated playlists, where hiding a track the user added
+// themselves would be wrong).
 // M7 #352 slice 2: system-generated playlist queries.
 // Active = had a play in the last 7 days. The cron iterates this list.
 func (q *Queries) ListActiveUsersForSystemPlaylists(ctx context.Context) ([]pgtype.UUID, error) {
@@ -298,7 +306,7 @@ recent7 AS (
            COUNT(*) FILTER (WHERE pe.was_skipped = false) AS play_count,
            0 AS tier
       FROM play_events pe
-      JOIN tracks t ON t.id = pe.track_id
+      JOIN tracks t ON t.id = pe.track_id AND t.missing_since IS NULL
      WHERE pe.user_id = $1
        AND pe.started_at > now() - INTERVAL '7 days'
        AND t.artist_id IS NOT NULL
@@ -309,7 +317,7 @@ recent30 AS (
            COUNT(*) FILTER (WHERE pe.was_skipped = false) AS play_count,
            1 AS tier
       FROM play_events pe
-      JOIN tracks t ON t.id = pe.track_id
+      JOIN tracks t ON t.id = pe.track_id AND t.missing_since IS NULL
      WHERE pe.user_id = $1
        AND pe.started_at > now() - INTERVAL '30 days'
        AND t.artist_id IS NOT NULL
@@ -320,7 +328,7 @@ alltime AS (
            COUNT(*) FILTER (WHERE pe.was_skipped = false) AS play_count,
            2 AS tier
       FROM play_events pe
-      JOIN tracks t ON t.id = pe.track_id
+      JOIN tracks t ON t.id = pe.track_id AND t.missing_since IS NULL
      WHERE pe.user_id = $1
        AND t.artist_id IS NOT NULL
      GROUP BY t.artist_id
@@ -432,7 +440,7 @@ const pickTopPlayedTrackForArtistByUser = `-- name: PickTopPlayedTrackForArtistB
 SELECT COALESCE(
     (SELECT t.id
        FROM play_events pe
-       JOIN tracks t ON t.id = pe.track_id
+       JOIN tracks t ON t.id = pe.track_id AND t.missing_since IS NULL
       WHERE pe.user_id = $1
         AND t.artist_id = $2
         AND pe.started_at > now() - INTERVAL '7 days'
@@ -472,7 +480,7 @@ const pickTopPlayedTracksForUser = `-- name: PickTopPlayedTracksForUser :many
 WITH recent AS (
     SELECT t.id, COUNT(*) AS c, 0 AS tier
       FROM play_events pe
-      JOIN tracks t ON t.id = pe.track_id
+      JOIN tracks t ON t.id = pe.track_id AND t.missing_since IS NULL
      WHERE pe.user_id = $1
        AND pe.started_at > now() - INTERVAL '30 days'
        AND pe.was_skipped = false
@@ -481,7 +489,7 @@ WITH recent AS (
 alltime AS (
     SELECT t.id, COUNT(*) AS c, 1 AS tier
       FROM play_events pe
-      JOIN tracks t ON t.id = pe.track_id
+      JOIN tracks t ON t.id = pe.track_id AND t.missing_since IS NULL
      WHERE pe.user_id = $1
        AND pe.was_skipped = false
      GROUP BY t.id