feat(conversion): add EPUB to KEPUB conversion service with kepubify

- Install kepubify binary in Dockerfile for on-the-fly conversion
- Add conversion service with caching layer (24hr TTL)
- Support KEPUB downloads through OPDS endpoint
- Cache converted files to reduce processing overhead
- Add environment configuration for cache directory and tool path

internal/services/conversion_service_test.go

@@ -0,0 +1,69 @@
+package services
+
+import (
+	"crypto/sha256"
+	"encoding/hex"
+	"os"
+	"path/filepath"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+func TestCalculateSHA256(t *testing.T) {
+	service := &ConversionService{}
+
+	dir := t.TempDir()
+	testFile := filepath.Join(dir, "test.epub")
+
+	content := "test content for hash calculation"
+	err := os.WriteFile(testFile, []byte(content), 0644)
+	require.NoError(t, err)
+
+	hash, err := service.calculateSHA256(testFile)
+	require.NoError(t, err)
+
+	expectedHash := sha256.New()
+	expectedHash.Write([]byte(content))
+	expectedHashStr := hex.EncodeToString(expectedHash.Sum(nil))
+
+	assert.Equal(t, expectedHashStr, hash)
+	assert.Len(t, hash, 64)
+}
+
+func TestConvertEPUBToKEPUBCreatesCacheDir(t *testing.T) {
+	cacheDir := filepath.Join(t.TempDir(), "cache", "kepub")
+	_ = NewConversionService(nil, cacheDir)
+
+	assert.NoFileExists(t, cacheDir, "Cache directory should not exist initially")
+
+	err := os.MkdirAll(cacheDir, 0755)
+	require.NoError(t, err, "Should be able to create cache directory")
+
+	assert.DirExists(t, cacheDir, "Cache directory should exist after creation")
+}
+
+func TestConvertedKEPUBStructure(t *testing.T) {
+	validHash := "abc123def456789abc123def456789abc123def456789abc123def456789abcd"
+	result := &ConvertedKEPUB{
+		Path:   "/path/to/book.kepub.epub",
+		SHA256: validHash,
+		Cached: false,
+	}
+
+	assert.NotEmpty(t, result.Path)
+	assert.NotEmpty(t, result.SHA256)
+	assert.False(t, result.Cached)
+	assert.Len(t, result.SHA256, 64)
+}
+
+func TestConversionServiceDefaults(t *testing.T) {
+	cacheDir := t.TempDir()
+	service := NewConversionService(nil, cacheDir)
+
+	assert.NotNil(t, service)
+	assert.Equal(t, cacheDir, service.cacheDir)
+	assert.Equal(t, "/usr/bin/kepubify", service.conversionTool)
+	assert.Equal(t, int64(24*3600*1000000000), service.conversionCacheTTL.Nanoseconds(), "Default TTL should be 24 hours")
+}