fix(scanner): read multi-value genre frames correctly — #2499

dhowden/tag's readTFrame splits ID3v2 null-separated multi-value text
frames and rejoins them with the EMPTY string, so a file tagged
"Alternative Rock" + "Rock" was stored as "Alternative RockRock". It also
leaves bare numeric ID3v1 references unresolved, which is why the
library showed genres like "4017" and "526617".

This corrupted more than the browse axis added in #367: taste_profile.sql
reads tracks.genre directly, so the welded tokens were entering the taste
profile's tag vocabulary, and recommendation.sql/discover.sql were
comparing them as single opaque tags. Genre counts were wrong everywhere.

ffprobe is not a fix — ffmpeg's read_ttag calls decode_str once with no
loop, keeping only the first value. Truncating multi-genre tags would
blunt the similarity signal genre mainly feeds. So the TCON frame is now
parsed directly (ID3v2.2/2.3/2.4, all four text encodings, per-frame and
tag-level unsynchronisation, numeric and parenthesised ID3v1 references);
everything else still comes from dhowden/tag. Values are stored
";"-delimited, which the read side already splits on, so no query changes.

Existing rows are repaired without an operator-run rebuild: migration
0054 adds tracks.tag_read_version DEFAULT 0, below the scanner's current
tagReadVersion, so the next scan re-reads tags it would otherwise skip on
mtime. Such a re-read reuses the stored duration instead of re-running
ffprobe, keeping a repair pass tag-read-bound rather than one fork+exec
per file. Bumping the constant is how a future extraction fix reaches an
existing library.

Only ID3v2 is in scope — dhowden welds nowhere else. The Vorbis/MP4
repeated-field question is #2500, unproven and deliberately not built.

internal/db/migrations/0054_track_tag_read_version.down.sql

@@ -0,0 +1,2 @@
+ALTER TABLE tracks
+    DROP COLUMN tag_read_version;

internal/db/migrations/0054_track_tag_read_version.up.sql

@@ -0,0 +1,15 @@
+-- Records which version of the scanner's tag-extraction logic last wrote a
+-- track's tag-derived columns (#2499).
+--
+-- DEFAULT 0 is the point of this migration: every existing row lands below the
+-- scanner's current library.tagReadVersion, so the next scan re-reads its tags
+-- instead of short-circuiting on the mtime check. That repairs genre values the
+-- old reader welded together ("Alternative Rock" + "Rock" -> "Alternative
+-- RockRock") without asking the operator to wipe and rebuild the library.
+--
+-- Bump library.tagReadVersion in Go — not this default — whenever a tag
+-- extraction fix needs to reach already-indexed files. That makes tag repairs a
+-- self-healing scan rather than a manual full rebuild, which is why this is a
+-- version number and not a boolean "needs_reread" flag.
+ALTER TABLE tracks
+    ADD COLUMN tag_read_version smallint NOT NULL DEFAULT 0;