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/search_cache_test.go

@@ -0,0 +1,67 @@
+package backend
+
+import (
+	"context"
+	"errors"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+func TestCleanSearchTerm(t *testing.T) {
+	tests := map[string]string{
+		"MGS4":          "mgs4",
+		"  test  ":      "test",
+		"Test 123":      "test123",
+		"A+B":           "aplusb",
+		"Mario & Luigi": "marioandluigi",
+		"É":             "e",
+		"It's":          "it's",
+		"Rock 'n Roll":  "rockandroll",
+		"":              "",
+	}
+	for input, expected := range tests {
+		assert.Equal(t, expected, CleanSearchTerm(input), input)
+	}
+}
+
+func TestMatchesSubsequence(t *testing.T) {
+	assert.True(t, MatchesSubsequence("", "anything"))
+	assert.True(t, MatchesSubsequence("ff7", "finalfantasy7"))
+	assert.True(t, MatchesSubsequence("lumière", "clairobscur-lumière"))
+	assert.False(t, MatchesSubsequence("7ff", "finalfantasy7"))
+	// Characters that are special in regular expressions are matched literally.
+	for _, term := range []string{"(", "[", "*", "\\", "a(b", ".*"} {
+		assert.NotPanics(t, func() { MatchesSubsequence(term, "plain text") }, term)
+		assert.False(t, MatchesSubsequence(term, "plain text"), term)
+	}
+	assert.True(t, MatchesSubsequence("(", "game (remake)"))
+}
+
+func TestSearchCache(t *testing.T) {
+	names := []string{"Final Fantasy VII"}
+	calls := 0
+	cache := NewSearchCache(func(context.Context) ([]string, error) {
+		calls++
+		return names, nil
+	})
+	ctx := context.Background()
+
+	data, err := cache.Get(ctx)
+	require.NoError(t, err)
+	assert.Equal(t, []SoundtrackData{{Original: "Final Fantasy VII", Lowercase: "final fantasy vii", Cleaned: "finalfantasyvii"}}, data)
+	_, _ = cache.Get(ctx)
+	assert.Equal(t, 1, calls, "data is cached")
+
+	names = []string{"Chrono Trigger"}
+	require.NoError(t, cache.Reload(ctx))
+	data, _ = cache.Get(ctx)
+	assert.Equal(t, "Chrono Trigger", data[0].Original)
+}
+
+func TestSearchCacheLoadError(t *testing.T) {
+	cache := NewSearchCache(func(context.Context) ([]string, error) { return nil, errors.New("db down") })
+	_, err := cache.Get(context.Background())
+	assert.Error(t, err)
+}