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/main_integration_test.go

@@ -0,0 +1,16 @@
+//go:build integration
+
+package db_test
+
+import (
+	"os"
+	"testing"
+
+	"music-server/internal/logging"
+	"music-server/internal/testutil/pgtest"
+)
+
+func TestMain(m *testing.M) {
+	logging.Init("error", false)
+	os.Exit(pgtest.Main(m))
+}