feat(playlists): Service with CRUD methods (M7 #352 slice 1)

Create / Get / List / Update / Delete with the visibility model from
the spec: private by default, owner-only mutations, public read for
non-owners. Update uses sqlc's CASE-WHEN-flag pattern for PATCH-style
partial updates without writing N variants.

Delete cleans up the cached cover file from disk best-effort. Track
operations (Append/Remove/Reorder) and the collage generator land
in subsequent tasks.

Also adds playlists + playlist_tracks to dbtest.ResetDB's truncate
list so integration tests in this package start from a clean state.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

internal/playlists/service_test_helpers_test.go

@@ -0,0 +1,92 @@
+package playlists_test
+
+import (
+	"context"
+	"crypto/rand"
+	"io"
+	"log/slog"
+	"os"
+	"testing"
+
+	"github.com/jackc/pgx/v5/pgtype"
+	"github.com/jackc/pgx/v5/pgxpool"
+
+	"git.fabledsword.com/bvandeusen/minstrel/internal/db"
+	"git.fabledsword.com/bvandeusen/minstrel/internal/db/dbq"
+	"git.fabledsword.com/bvandeusen/minstrel/internal/dbtest"
+)
+
+// 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
+// the env var isn't set or -short is passed.
+func newPool(t *testing.T) *pgxpool.Pool {
+	t.Helper()
+	if testing.Short() {
+		t.Skip("skipping integration test in -short mode")
+	}
+	dsn := os.Getenv("MINSTREL_TEST_DATABASE_URL")
+	if dsn == "" {
+		t.Skip("MINSTREL_TEST_DATABASE_URL not set")
+	}
+	if err := db.Migrate(dsn, slog.New(slog.NewTextHandler(io.Discard, nil))); err != nil {
+		t.Fatalf("migrate: %v", err)
+	}
+	pool, err := pgxpool.New(context.Background(), dsn)
+	if err != nil {
+		t.Fatalf("pool: %v", err)
+	}
+	t.Cleanup(pool.Close)
+	dbtest.ResetDB(t, pool)
+	return pool
+}
+
+// seedUser inserts a `test-`-prefixed user (so dbtest.ResetDB can
+// clean it without touching the operator's admin row) and returns
+// the persisted user record.
+func seedUser(t *testing.T, pool *pgxpool.Pool, name string) dbq.User {
+	t.Helper()
+	u, err := dbq.New(pool).CreateUser(context.Background(), dbq.CreateUserParams{
+		Username:     dbtest.TestUserPrefix + name,
+		PasswordHash: "x",
+		ApiToken:     name + "-token",
+		IsAdmin:      false,
+	})
+	if err != nil {
+		t.Fatalf("seed user %s: %v", name, err)
+	}
+	return u
+}
+
+// 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 {
+	var u pgtype.UUID
+	if _, err := rand.Read(u.Bytes[:]); err != nil {
+		panic(err)
+	}
+	u.Valid = true
+	return u
+}
+
+// uuidString renders a pgtype.UUID as the canonical 8-4-4-4-12 form
+// without depending on a third-party UUID package — used in tests
+// that build a filename from a playlist id.
+func uuidString(u pgtype.UUID) string {
+	if !u.Valid {
+		return ""
+	}
+	const hex = "0123456789abcdef"
+	out := make([]byte, 36)
+	j := 0
+	for i, x := range u.Bytes {
+		if i == 4 || i == 6 || i == 8 || i == 10 {
+			out[j] = '-'
+			j++
+		}
+		out[j] = hex[x>>4]
+		out[j+1] = hex[x&0x0f]
+		j += 2
+	}
+	return string(out)
+}