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/backend/releases_test.go

@@ -0,0 +1,81 @@
+package backend
+
+import (
+	"context"
+	"encoding/json"
+	"net/http"
+	"net/http/httptest"
+	"sync/atomic"
+	"testing"
+	"time"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+func newGitea(t *testing.T, status int) (*httptest.Server, *atomic.Int32) {
+	var calls atomic.Int32
+	server := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		calls.Add(1)
+		w.WriteHeader(status)
+		json.NewEncoder(w).Encode(giteaRelease{
+			Id:   1,
+			Name: "v1.2.3",
+			Assets: []giteaAsset{
+				{Name: "MusicPlayer.exe", DownloadUrl: "https://example.com/MusicPlayer.exe"},
+				{Name: "MusicPlayer.x86_64", DownloadUrl: "https://example.com/MusicPlayer.x86_64"},
+			},
+		})
+	}))
+	t.Cleanup(server.Close)
+	return server, &calls
+}
+
+func TestReleaseService(t *testing.T) {
+	server, _ := newGitea(t, http.StatusOK)
+	r := NewReleaseService(server.URL)
+	ctx := context.Background()
+
+	version, err := r.LatestVersion(ctx)
+	require.NoError(t, err)
+	assert.Equal(t, "v1.2.3", version)
+
+	names, err := r.AssetNames(ctx)
+	require.NoError(t, err)
+	assert.Equal(t, []string{"MusicPlayer.exe", "MusicPlayer.x86_64"}, names)
+
+	url, err := r.AssetURL(ctx, ".exe")
+	require.NoError(t, err)
+	assert.Equal(t, "https://example.com/MusicPlayer.exe", url)
+
+	_, err = r.AssetURL(ctx, ".dmg")
+	assert.ErrorIs(t, err, ErrNotFound)
+}
+
+func TestReleaseServiceCachesRelease(t *testing.T) {
+	server, calls := newGitea(t, http.StatusOK)
+	r := NewReleaseService(server.URL)
+	now := time.Now()
+	r.now = func() time.Time { return now }
+	ctx := context.Background()
+
+	for range 3 {
+		_, err := r.LatestVersion(ctx)
+		require.NoError(t, err)
+	}
+	assert.Equal(t, int32(1), calls.Load())
+
+	now = now.Add(releaseCacheTTL + time.Second)
+	_, err := r.LatestVersion(ctx)
+	require.NoError(t, err)
+	assert.Equal(t, int32(2), calls.Load())
+}
+
+func TestReleaseServiceUpstreamErrors(t *testing.T) {
+	server, _ := newGitea(t, http.StatusInternalServerError)
+	_, err := NewReleaseService(server.URL).LatestVersion(context.Background())
+	assert.ErrorIs(t, err, ErrUpstream)
+
+	_, err = NewReleaseService("http://127.0.0.1:1").AssetNames(context.Background())
+	assert.ErrorIs(t, err, ErrUpstream)
+}