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/tracks.sql

@@ -137,3 +137,34 @@ RETURNING id, album_id, artist_id, file_path, mbid;
 -- Batched lookup used by /api/library/sync to hydrate upsert payloads
 -- (#357). Mirror of GetArtistsByIDs.
 SELECT * FROM tracks WHERE id = ANY($1::uuid[]);
+
+-- name: ListTrackPathsForReconcile :many
+-- Every row's path + current missing mark, for the scanner's reconcile pass
+-- (#2523). Deliberately unfiltered and unpaged: reconcile has to compare the
+-- WHOLE table against what the walk saw, and a filtered subset would let rows
+-- outside it drift forever. Three narrow columns keep it cheap even on a
+-- library of a few hundred thousand tracks.
+SELECT id, file_path, missing_since FROM tracks;
+
+-- name: MarkTracksMissing :execrows
+-- Marks rows whose file the walk did not see. `missing_since IS NULL` in the
+-- predicate makes this idempotent: a row already marked keeps its ORIGINAL
+-- timestamp, so "how long has it been gone" survives repeated scans. Losing
+-- that would make any age-based cleanup policy meaningless.
+--
+-- updated_at is deliberately NOT touched. It tracks content changes and gates
+-- the scanner's mtime skip; moving it here would make a returning file look
+-- newer than its own mtime and stop its tags being re-read.
+UPDATE tracks
+   SET missing_since = now()
+ WHERE id = ANY(sqlc.arg(ids)::uuid[])
+   AND missing_since IS NULL;
+
+-- name: ClearTracksMissing :execrows
+-- Clears the mark on rows whose file is back. Runs independently of the mtime
+-- skip check, so a file that reappears unchanged is un-marked even though the
+-- scanner skips re-reading its tags.
+UPDATE tracks
+   SET missing_since = NULL
+ WHERE id = ANY(sqlc.arg(ids)::uuid[])
+   AND missing_since IS NOT NULL;