fix(tagwriter): declare the track and disc totals when tagging

An album the user holds 2 of 10 tracks of showed a green tick reading
"is in your library", and the mechanism was our own writer. tagwriter
wrote track and disc *numbers* and dropped the totals, so autotagging a
folder made the release MBID-matched -- which is what earns the tick --
while erasing the one field GetAlbumCompleteness reads. The evidence
for "2 of 10" was destroyed by the act that produced the tick.

FieldTotalTracks and FieldTotalDiscs are written as the ID3 "n/N" form
and as Vorbis TRACKTOTAL/DISCTOTAL; the autotag apply pass and the
download importer fill them from the release's own tracklist; and
dbsync persists the track total to audio_files.total_tracks so the
album page agrees with the file without waiting for a rescan.

Five things about it are load-bearing, and four fail silently:

- The total is per *disc*, not per release, because that is what the
  tag form declares and what GetAlbumCompleteness sums per disc. A
  release total on every file multiplies a two-disc album's expectation
  by two, which no library can satisfy. backend/tagtotals is that
  derivation once, since the two callers must not import the writer or
  each other.
- The Vorbis names are TRACKTOTAL and DISCTOTAL and no other spelling.
  dhowden/tag reads exactly those two keys, so TOTALTRACKS -- which
  xiph lists and several taggers write -- or a "1/12" packed into
  TRACKNUMBER writes successfully and reads back as no total at all.
  The tests therefore assert the round trip through the reader the scan
  uses, not through the bytes.
- ID3's number and total share one frame, so writing either alone must
  read the other off the existing tag or discard it. A total with no
  number is not written: "/12" parses as track 0.
- The totals are written unconditionally rather than on a diff. The
  case this exists for is a file declaring no total at all, which
  compares equal to nothing and is exactly what a "only if it changed"
  guard skips.
- A single-track download is not totalled. A RecordingMBID anchor
  resolves Expected to that one track, so the same code would tag a
  track off a twelve-track album "1 of 1" -- and a declared total
  outranks the catalog total that would have answered correctly.

autotag's field constants are a second copy of tagwriter's, deliberately
so autotag stays out of the write pipeline's import graph. A key that
drifts neither fails to compile nor fails to write -- the writer simply
finds nothing under the name it looks for -- so autotagservice, the one
package importing both, now pins them.

Steps 2 and 3 of the issue stay open under #38: the catalog fallback
already landed as completenessAnswer(), and the badge call-site audit is
the part that overlaps it.

Closes #16

backend/autotag/apply.go

@@ -8,6 +8,7 @@ import (
 	"log/slog"
 
 	"yellowjacket/backend/database/sql/sqlcgen"
+	"yellowjacket/backend/tagtotals"
 )
 
 // TagChanges mirrors tagwriter.TagChanges — redefined here so the
@@ -28,6 +29,8 @@ const (
 	FieldYear        = "year"
 	FieldTrackNumber = "track_number"
 	FieldDiscNumber  = "disc_number"
+	FieldTotalTracks = "total_tracks"
+	FieldTotalDiscs  = "total_discs"
 	FieldCoverArt    = "cover_art"
 )
 
@@ -418,5 +421,32 @@ func buildChanges(
 		changes[FieldDiscNumber] = track.DiscNumber
 	}
 
+	// The totals are what says "2 of 10" rather than a bare tick, and
+	// dropping them here is what made autotagging an album *erase* the
+	// evidence: the release becomes MBID-matched while the field
+	// GetAlbumCompleteness reads stays absent.
+	//
+	// They are written unconditionally where the candidate has a
+	// tracklist, not only when they differ from the local value, because
+	// the common case is a file that declares no total at all -- which
+	// compares equal to nothing and would be skipped by a diff guard.
+	if tracks, discs := tagtotals.For(
+		candidatePositions(cand), track.DiscNumber,
+	); tracks > 0 {
+		changes[FieldTotalTracks] = tracks
+		changes[FieldTotalDiscs] = discs
+	}
+
 	return changes
 }
+
+// candidatePositions is the candidate's tracklist as bare positions.
+func candidatePositions(cand Candidate) []tagtotals.Position {
+	out := make([]tagtotals.Position, 0, len(cand.Tracks))
+
+	for _, t := range cand.Tracks {
+		out = append(out, tagtotals.Position{Disc: t.DiscNumber, Track: t.Position})
+	}
+
+	return out
+}

backend/autotag/buildchanges_test.go

@@ -0,0 +1,90 @@
+package autotag
+
+import "testing"
+
+// Autotagging an album used to *erase* the evidence that says "2 of 10":
+// the release became MBID-matched while the totals the files declared
+// went unwritten, so the album page showed a plain tick.  These pin the
+// two halves of the fix that are easy to get wrong silently.
+func TestBuildChanges_Totals(t *testing.T) {
+	t.Parallel()
+
+	twoDiscs := Candidate{
+		Tracks: []CandidateTrack{
+			{DiscNumber: 1, Position: 1},
+			{DiscNumber: 1, Position: 2},
+			{DiscNumber: 2, Position: 1},
+			{DiscNumber: 2, Position: 2},
+			{DiscNumber: 2, Position: 3},
+		},
+	}
+
+	tests := []struct {
+		name       string
+		cand       Candidate
+		local      LocalTrack
+		track      CandidateTrack
+		wantTracks any
+		wantDiscs  any
+	}{
+		{
+			// The common case, and the one a diff guard would skip: the
+			// file declares no total at all, so the total "has not
+			// changed" and would never be written.
+			name: "a file with no total gets one",
+			cand: Candidate{Tracks: []CandidateTrack{
+				{Position: 1}, {Position: 2}, {Position: 3},
+			}},
+			local:      LocalTrack{TrackNumber: 1},
+			track:      CandidateTrack{Position: 1},
+			wantTracks: 3,
+			wantDiscs:  1,
+		},
+		{
+			// 5 here would be the release's track count.  Summed once
+			// per disc by GetAlbumCompleteness that claims a ten-track
+			// expectation for a five-track album, which no library can
+			// ever satisfy.
+			name:       "a multi-disc release totals the track's own disc",
+			cand:       twoDiscs,
+			local:      LocalTrack{},
+			track:      CandidateTrack{DiscNumber: 2, Position: 1},
+			wantTracks: 3,
+			wantDiscs:  2,
+		},
+		{
+			name:       "the other disc gets its own total",
+			cand:       twoDiscs,
+			local:      LocalTrack{},
+			track:      CandidateTrack{DiscNumber: 1, Position: 1},
+			wantTracks: 2,
+			wantDiscs:  2,
+		},
+		{
+			// A candidate with no tracklist knows nothing, and writing
+			// a zero would claim it did.
+			name:       "a candidate with no tracklist writes no total",
+			cand:       Candidate{},
+			local:      LocalTrack{},
+			track:      CandidateTrack{Position: 1},
+			wantTracks: nil,
+			wantDiscs:  nil,
+		},
+	}
+
+	for _, tc := range tests {
+		t.Run(tc.name, func(t *testing.T) {
+			t.Parallel()
+
+			changes := buildChanges(tc.local, tc.cand, tc.track)
+
+			if got := changes[FieldTotalTracks]; got != tc.wantTracks {
+				t.Errorf("%s: got %v, want %v", FieldTotalTracks, got, tc.wantTracks)
+			}
+
+			if got := changes[FieldTotalDiscs]; got != tc.wantDiscs {
+				t.Errorf("%s: got %v, want %v", FieldTotalDiscs, got, tc.wantDiscs)
+			}
+		})
+	}
+}