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

Dockerfile

@@ -42,6 +42,11 @@ RUN CGO_ENABLED=0 GOOS=linux go build -a -installsuffix cgo -o main ./cmd/server
 FROM alpine:latest
 
 RUN apk --no-cache add ca-certificates curl
+
+# Install kepubify for EPUB→KEPUB conversion
+RUN wget -O /usr/bin/kepubify https://github.com/pgaskin/kepubify/releases/latest/download/kepubify-linux-64bit \
+    && chmod +x /usr/bin/kepubify
+
 WORKDIR /root/
 
 # Copy the binary from builder stage