Fix hot cache to include ContentType and SizeBytes

Hot cache entries now store all data needed to serve a cache hit
without any database access:
- OutputHash (for file lookup)
- ContentType (for Content-Type header)
- SizeBytes (for Content-Length header)

Previously hot cache only stored OutputHash, causing empty
Content-Type headers on cached WebP responses.

Makefile

@@ -28,9 +28,11 @@ test:
 	go test -v ./...
 
 # Build the binary
-build:
+build: ./bin/pixad
+
+./bin/pixad: ./internal/*/*.go ./cmd/pixad/*.go
 	@echo "Building pixad..."
-	go build -ldflags "$(LDFLAGS)" -o bin/pixad ./cmd/pixad
+	go build -ldflags "$(LDFLAGS)" -o $@ ./cmd/pixad
 
 # Clean build artifacts
 clean: