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.

Dockerfile

@@ -7,6 +7,7 @@ ARG VERSION=dev
 RUN apk add --no-cache \
     build-base \
     vips-dev \
+    libheif-dev \
     pkgconfig
 
 WORKDIR /src
@@ -27,6 +28,7 @@ FROM alpine:3.21
 # Install runtime dependencies only
 RUN apk add --no-cache \
     vips \
+    libheif \
     ca-certificates \
     tzdata