Switch to govips for native CGO image processing

- Replace gen2brain/avif, gen2brain/webp, disintegration/imaging with govips
- govips uses libvips via CGO for fast native image processing
- Add libheif-dev to Dockerfile for AVIF support
- Add docker-test Makefile target for running tests in Docker
- Update processor.go to use vips API for decode, resize, encode
- Add TestMain to initialize/shutdown vips in tests
- Remove WASM-based libraries (gen2brain) in favor of native codecs

Performance improvement: AVIF encoding now uses native libheif instead of
WASM, significantly reducing encoding time for large images.

Makefile

@@ -1,4 +1,4 @@
-.PHONY: check lint test fmt build clean docker
+.PHONY: check lint test fmt build clean docker docker-test
 
 VERSION := $(shell git describe --tags --always --dirty 2>/dev/null || echo "dev")
 LDFLAGS := -X main.Version=$(VERSION)
@@ -41,3 +41,8 @@ clean:
 # Build Docker image
 docker:
 	docker build --build-arg VERSION=$(VERSION) -t pixad:$(VERSION) -t pixad:latest .
+
+# Run tests in Docker (needed for CGO/libvips)
+docker-test:
+	docker build --target builder --build-arg VERSION=$(VERSION) -t pixad-builder .
+	docker run --rm pixad-builder sh -c "CGO_ENABLED=1 GOTOOLCHAIN=auto go test -v ./..."