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/characters_test.go

@@ -0,0 +1,58 @@
+package backend
+
+import (
+	"io"
+	"path/filepath"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+func newCharacterService(t *testing.T) *CharacterService {
+	t.Helper()
+	base := t.TempDir()
+	writeFile(t, filepath.Join(base, "secret.png"), "outside")
+	dir := filepath.Join(base, "characters")
+	writeFile(t, filepath.Join(dir, "Adol.png"), "adol")
+	writeFile(t, filepath.Join(dir, "Aloy.JPG"), "aloy")
+	writeFile(t, filepath.Join(dir, "notes.txt"), "text")
+	writeFile(t, filepath.Join(dir, "sub", "Hidden.png"), "hidden")
+
+	c, err := NewCharacterService(dir)
+	require.NoError(t, err)
+	t.Cleanup(func() { c.Close() })
+	return c
+}
+
+func TestCharacterList(t *testing.T) {
+	characters, err := newCharacterService(t).List()
+	require.NoError(t, err)
+	assert.Equal(t, []string{"Adol.png", "Aloy.JPG"}, characters)
+}
+
+func TestCharacterOpen(t *testing.T) {
+	c := newCharacterService(t)
+
+	f, err := c.Open("Adol.png")
+	require.NoError(t, err)
+	content, _ := io.ReadAll(f)
+	f.Close()
+	assert.Equal(t, "adol", string(content))
+
+	_, err = c.Open("Missing.png")
+	assert.ErrorIs(t, err, ErrNotFound)
+}
+
+func TestCharacterOpenRejectsPathTraversal(t *testing.T) {
+	c := newCharacterService(t)
+	for _, name := range []string{"", "../secret.png", "sub/Hidden.png", "/etc/passwd", "..", "notes.txt"} {
+		_, err := c.Open(name)
+		assert.ErrorIs(t, err, ErrInvalidName, name)
+	}
+}
+
+func TestNewCharacterServiceMissingDir(t *testing.T) {
+	_, err := NewCharacterService(filepath.Join(t.TempDir(), "missing"))
+	assert.Error(t, err)
+}