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

.env.example

@@ -17,4 +17,12 @@ TEST_MODE=false
 RATE_LIMIT_ENABLED=true
 # Number of requests allowed per minute per IP (default: 10)
 # Increase for integration testing (e.g., 1000)
-REQUESTS_PER_MINUTE=10
+REQUESTS_PER_MINUTE=10
+
+# Conversion Service Configuration
+# Directory for caching converted files (KEPUB, etc.)
+BOOKMANN_CONVERSION_CACHE_DIR=/var/bookmann/cache/kepub
+# Conversion tool to use (kepubify or ebook-convert)
+BOOKMANN_CONVERSION_TOOL=/usr/bin/kepubify
+# Cache TTL for converted files (e.g., 24h, 12h, 48h)
+BOOKMANN_CONVERSION_CACHE_TTL=24h