feat(playlists): track operations + cover-collage generator

AppendTracks / RemoveTrack / Reorder run in single transactions and
update the denormalized rollups (track_count, duration_sec). Reorder
uses a +10000 offset to bump every row out of the position range
before writing the new positions, sidestepping PK conflicts during
rewrite.

GenerateCollage composes a 600x600 JPEG from the first 4 album
covers, with a slate-tinted fallback for missing cells. Synchronous,
called inline after every mutating operation. SVG-rasterized fallback
is a follow-up — slice 1 ships with a solid placeholder.

internal/playlists/service_test_helpers_test.go

@@ -3,9 +3,12 @@ package playlists_test
 import (
 	"context"
 	"crypto/rand"
+	"fmt"
 	"io"
 	"log/slog"
 	"os"
+	"path/filepath"
+	"sync/atomic"
 	"testing"
 
 	"github.com/jackc/pgx/v5/pgtype"
@@ -16,6 +19,12 @@ import (
 	"git.fabledsword.com/bvandeusen/minstrel/internal/dbtest"
 )
 
+// seedTrackCounter ensures every seedTrack call gets a unique file_path
+// even when title + artist repeat. Tracks dedupe by file_path so we
+// must vary it; using t.TempDir() alone isn't enough because some
+// tests seed multiple tracks within one test function.
+var seedTrackCounter uint64
+
 // newPool migrates a fresh schema against MINSTREL_TEST_DATABASE_URL
 // and returns a connection pool with all data tables wiped (test
 // users only — see internal/dbtest/reset.go for the why). Skips when
@@ -58,6 +67,40 @@ func seedUser(t *testing.T, pool *pgxpool.Pool, name string) dbq.User {
 	return u
 }
 
+// seedTrack creates an artist + album + track triple suitable for
+// playlist track-list mutation tests. Each call produces a fresh
+// track row with a unique file_path; artist and album are not
+// deduplicated across calls (mbid-less upsert), but that's fine for
+// these tests — playlist snapshots just capture the strings.
+func seedTrack(t *testing.T, pool *pgxpool.Pool, title, artist string) dbq.Track {
+	t.Helper()
+	q := dbq.New(pool)
+	a, err := q.UpsertArtist(context.Background(), dbq.UpsertArtistParams{
+		Name: artist, SortName: artist,
+	})
+	if err != nil {
+		t.Fatalf("seed artist: %v", err)
+	}
+	al, err := q.UpsertAlbum(context.Background(), dbq.UpsertAlbumParams{
+		Title: artist + " - Album", SortTitle: artist + " - Album",
+		ArtistID: a.ID,
+	})
+	if err != nil {
+		t.Fatalf("seed album: %v", err)
+	}
+	n := atomic.AddUint64(&seedTrackCounter, 1)
+	track, err := q.UpsertTrack(context.Background(), dbq.UpsertTrackParams{
+		Title: title, AlbumID: al.ID, ArtistID: a.ID,
+		DurationMs: 1000,
+		FilePath:   filepath.Join(t.TempDir(), fmt.Sprintf("%s-%d.mp3", title, n)),
+		FileSize:   100, FileFormat: "mp3",
+	})
+	if err != nil {
+		t.Fatalf("seed track: %v", err)
+	}
+	return track
+}
+
 // randomUUID returns a fresh valid pgtype.UUID. Used in tests that
 // need an id guaranteed-not-to-exist in the database.
 func randomUUID() pgtype.UUID {