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/queries/browse.sql

@@ -1,3 +1,15 @@
+-- Every query in this file filters `tracks.missing_since IS NULL` (#2523).
+-- A row whose file has vanished keeps its genre forever — the scanner walks the
+-- filesystem, so it never revisits a path that no longer exists — which is how
+-- pre-#2499 welded genres survived a full re-scan and kept showing in the index.
+-- Browsing is a way of finding something to play, so a track that cannot play
+-- should not shape it.
+--
+-- Year queries below join albums only and are deliberately left alone: an album
+-- is still a real release even if some of its tracks are gone. An album whose
+-- EVERY track is missing will linger on the year axis; that's a narrower case,
+-- tracked with the rest of the cleanup work.
+
 -- name: ListGenresWithCount :many
 -- Genre browse index (#367).
 --
@@ -22,6 +34,7 @@ SELECT trim(g.genre) AS genre, COUNT(DISTINCT tracks.id)::bigint AS track_count
 FROM tracks
 JOIN LATERAL regexp_split_to_table(coalesce(tracks.genre, ''), '[;,]') AS g(genre) ON true
 WHERE trim(g.genre) <> ''
+  AND tracks.missing_since IS NULL
 GROUP BY trim(g.genre)
 -- Ordered by the expression, not the output alias: `ORDER BY genre` is
 -- ambiguous between the alias and tracks.genre, and sqlc rejects it.
@@ -41,6 +54,7 @@ WHERE EXISTS (
     FROM tracks
     JOIN LATERAL regexp_split_to_table(coalesce(tracks.genre, ''), '[;,]') AS g(genre) ON true
     WHERE tracks.album_id = albums.id
+      AND tracks.missing_since IS NULL
       AND trim(g.genre) = trim(sqlc.arg(genre)::text)
 )
 ORDER BY albums.sort_title, albums.id
@@ -56,6 +70,7 @@ WHERE EXISTS (
     FROM tracks
     JOIN LATERAL regexp_split_to_table(coalesce(tracks.genre, ''), '[;,]') AS g(genre) ON true
     WHERE tracks.album_id = albums.id
+      AND tracks.missing_since IS NULL
       AND trim(g.genre) = trim(sqlc.arg(genre)::text)
 );
 
@@ -96,6 +111,7 @@ SELECT DISTINCT trim(g.genre) AS genre
 FROM tracks
 JOIN LATERAL regexp_split_to_table(coalesce(tracks.genre, ''), '[;,]') AS g(genre) ON true
 WHERE tracks.album_id = $1 AND trim(g.genre) <> ''
+  AND tracks.missing_since IS NULL
 ORDER BY trim(g.genre);
 
 -- name: ListGenresForArtist :many
@@ -106,4 +122,5 @@ SELECT DISTINCT trim(g.genre) AS genre
 FROM tracks
 JOIN LATERAL regexp_split_to_table(coalesce(tracks.genre, ''), '[;,]') AS g(genre) ON true
 WHERE tracks.artist_id = $1 AND trim(g.genre) <> ''
+  AND tracks.missing_since IS NULL
 ORDER BY trim(g.genre);