Rewrite tests to run without a database

- Unit tests use an in-memory fake database (internal/testutil/fakedb),
  temp folders and httptest; `go test ./...` needs no Docker or env vars
- Integration tests (build tag "integration") start PostgreSQL with
  testcontainers: migrations step by step, SQL dumps (skipped if
  missing) and a full sync against a real database
- Removed tests that only checked copies of the logic or had empty
  bodies, the broken root integration_test.go, compose.test.yaml and
  .env.test with hardcoded paths
- testMusic MP3s replaced with tiny placeholders (sync never decodes
  audio), 194 MB -> 168 KB

internal/db/sql_dump_integration_test.go

@@ -0,0 +1,39 @@
+//go:build integration
+
+package db_test
+
+import (
+	"os"
+	"path/filepath"
+	"testing"
+
+	"music-server/internal/testutil/pgtest"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+// TestSQLDumps migrates real database dumps (music_*_dump.sql in the repository root, not committed).
+func TestSQLDumps(t *testing.T) {
+	for _, dump := range []string{"music_ref_dump.sql", "music_test_dump.sql", "music_prod_dump.sql"} {
+		t.Run(dump, func(t *testing.T) {
+			path := filepath.Join("..", "..", dump)
+			if _, err := os.Stat(path); err != nil {
+				t.Skipf("%s not found", dump)
+			}
+
+			pgtest.CreateRole(t, "sansan")
+			dbname := pgtest.NewDatabase(t)
+			pgtest.LoadSQLFile(t, dbname, path)
+			database := openDatabase(t, dbname)
+			soundtracksBefore := count(t, database, "SELECT COUNT(*) FROM game")
+			songsBefore := count(t, database, "SELECT COUNT(*) FROM song")
+
+			require.NoError(t, database.RunMigrations())
+
+			assert.Equal(t, soundtracksBefore, count(t, database, "SELECT COUNT(*) FROM soundtrack"))
+			assert.Equal(t, songsBefore, count(t, database, "SELECT COUNT(*) FROM song"))
+			assert.Equal(t, 0, count(t, database, "SELECT COUNT(*) FROM song WHERE uuid IS NULL"))
+		})
+	}
+}