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/config/config_test.go

@@ -0,0 +1,64 @@
+package config
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+func env(values map[string]string) func(string) string {
+	return func(key string) string { return values[key] }
+}
+
+func validEnv() map[string]string {
+	return map[string]string{
+		"DB_HOST":         "localhost",
+		"DB_PORT":         "5432",
+		"DB_USERNAME":     "user",
+		"DB_PASSWORD":     "pass",
+		"DB_NAME":         "music",
+		"MUSIC_PATH":      "/music",
+		"CHARACTERS_PATH": "/characters",
+	}
+}
+
+func TestLoadDefaults(t *testing.T) {
+	cfg, err := load(env(validEnv()))
+	require.NoError(t, err)
+	assert.Equal(t, 8080, cfg.Port)
+	assert.Equal(t, "info", cfg.LogLevel)
+	assert.Equal(t, "songs", cfg.SoundCheckPath)
+	assert.Equal(t, DefaultReleasesURL, cfg.ReleasesURL)
+	assert.Empty(t, cfg.TrustedProxies)
+}
+
+func TestLoadMissingRequired(t *testing.T) {
+	values := validEnv()
+	delete(values, "MUSIC_PATH")
+	delete(values, "DB_HOST")
+	_, err := load(env(values))
+	require.Error(t, err)
+	assert.Contains(t, err.Error(), "MUSIC_PATH is required")
+	assert.Contains(t, err.Error(), "DB_HOST is required")
+}
+
+func TestLoadTrustedProxies(t *testing.T) {
+	values := validEnv()
+	values["TRUSTED_PROXIES"] = "10.0.0.0/8, 172.16.0.0/12"
+	cfg, err := load(env(values))
+	require.NoError(t, err)
+	require.Len(t, cfg.TrustedProxies, 2)
+	assert.Equal(t, "10.0.0.0/8", cfg.TrustedProxies[0].String())
+
+	values["TRUSTED_PROXIES"] = "not-a-cidr"
+	_, err = load(env(values))
+	assert.ErrorContains(t, err, "TRUSTED_PROXIES")
+}
+
+func TestLoadInvalidPort(t *testing.T) {
+	values := validEnv()
+	values["PORT"] = "abc"
+	_, err := load(env(values))
+	assert.ErrorContains(t, err, "PORT")
+}