Install libvips JPEG XL support and require it at startup (part of #222)

script/bootstrap --cgo installs vips-jxl when the package manager it
uses is apk, as Alpine's vips package lacks JPEG XL support; the nix
and brew libvips include it, as does apt's from Debian 12 and Ubuntu
24.04 on. The runtime stage of the Dockerfile installs vips-jxl too.

imgcache.NewService calls the new imageprocessor.CheckJPEGXLSupport
before it builds the image processor and fails with its error, which
names the fix, so pixad does not start without the support. JPEG XL
becomes the default output later in the issue. New tests check the
support and save an image as JPEG XL with govips and load it back.

Model: opus-5-5

Dockerfile

@@ -22,8 +22,9 @@ FROM golang:1.25.4-alpine@sha256:d3f0cf7723f3429e3f9ed846243970b20a2de7bae6a5b66
 
 WORKDIR /src
 
-# script/bootstrap --cgo installs the build dependencies (a C compiler
-# and the libvips and libheif headers) and downloads the Go modules.
+# script/bootstrap --cgo installs the build dependencies (a C compiler,
+# the libvips and libheif headers, and libvips' JPEG XL support, which
+# the tests need) and downloads the Go modules.
 COPY script/ ./script/
 COPY go.mod go.sum ./
 RUN script/bootstrap --cgo
@@ -80,9 +81,11 @@ RUN version="${VERSION:-$(git describe --tags --always)}"; \
 # alpine:3.21, 2026-02-25
 FROM alpine:3.21@sha256:c3f8e73fdb79deaebaa2037150150191b9dcbfba68b4a46d70103204c53f4709
 
-# Install runtime dependencies only
+# Install runtime dependencies only. vips-jxl is libvips' JPEG XL
+# support, without which pixad does not start.
 RUN apk add --no-cache \
     vips \
+    vips-jxl \
     libheif \
     ca-certificates \
     tzdata \