Implement ETag, HEAD requests, and conditional requests

- Add ETag generation based on output content hash (first 16 chars)
- Add ContentLength to ImageResponse from cache
- Add LoadWithSize method to ContentStorage
- Add GetOutputWithSize method to Cache
- Handle HEAD requests returning headers only
- Handle If-None-Match conditional requests returning 304
- Register HEAD route for image proxy endpoint

internal/server/routes.go

@@ -53,6 +53,7 @@ func (s *Server) SetupRoutes() {
 	// Main image proxy route
 	// /v1/image/<host>/<path>/<width>x<height>.<format>
 	s.router.Get("/v1/image/*", s.h.HandleImage())
+	s.router.Head("/v1/image/*", s.h.HandleImage())
 
 	// Encrypted image URL route
 	s.router.Get("/v1/e/{token}", s.h.HandleImageEnc())