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/collage_test.go

@@ -0,0 +1,75 @@
+package playlists_test
+
+import (
+	"context"
+	"image"
+	_ "image/jpeg"
+	"os"
+	"path/filepath"
+	"testing"
+
+	"github.com/jackc/pgx/v5/pgtype"
+
+	"git.fabledsword.com/bvandeusen/minstrel/internal/playlists"
+)
+
+func TestGenerateCollage_EmptyPlaylist(t *testing.T) {
+	pool := newPool(t)
+	user := seedUser(t, pool, "alice")
+	dir := t.TempDir()
+	svc := playlists.NewService(pool, nil, dir)
+
+	pl, err := svc.Create(context.Background(), user.ID, "Empty", "", false)
+	if err != nil {
+		t.Fatalf("Create: %v", err)
+	}
+
+	relPath, err := playlists.GenerateCollage(context.Background(), pool, pl.ID, dir)
+	if err != nil {
+		t.Fatalf("GenerateCollage: %v", err)
+	}
+	if relPath != "" {
+		t.Errorf("relPath = %q, want \"\" for empty playlist", relPath)
+	}
+}
+
+func TestGenerateCollage_OneTrack(t *testing.T) {
+	pool := newPool(t)
+	user := seedUser(t, pool, "alice")
+	tk := seedTrack(t, pool, "Roygbiv", "BoC")
+	dir := t.TempDir()
+	svc := playlists.NewService(pool, nil, dir)
+
+	pl, err := svc.Create(context.Background(), user.ID, "One", "", false)
+	if err != nil {
+		t.Fatalf("Create: %v", err)
+	}
+	if err := svc.AppendTracks(context.Background(), user.ID, pl.ID, []pgtype.UUID{tk.ID}); err != nil {
+		t.Fatalf("AppendTracks: %v", err)
+	}
+
+	got, err := svc.Get(context.Background(), user.ID, pl.ID)
+	if err != nil {
+		t.Fatalf("Get: %v", err)
+	}
+	if got.CoverPath == nil || *got.CoverPath == "" {
+		t.Fatalf("CoverPath empty after append")
+	}
+
+	full := filepath.Join(dir, *got.CoverPath)
+	f, err := os.Open(full)
+	if err != nil {
+		t.Fatalf("open collage: %v", err)
+	}
+	defer f.Close()
+	img, format, err := image.Decode(f)
+	if err != nil {
+		t.Fatalf("decode collage: %v", err)
+	}
+	if format != "jpeg" {
+		t.Errorf("format = %q, want jpeg", format)
+	}
+	if img.Bounds().Dx() != 600 || img.Bounds().Dy() != 600 {
+		t.Errorf("dimensions = %dx%d, want 600x600", img.Bounds().Dx(), img.Bounds().Dy())
+	}
+}