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

TODO.md

@@ -30,6 +30,14 @@ P2: security: per-IP rate limiting on the image routes
 
 # Completed Steps
 
+- 2026-10-08 libvips' JPEG XL support is installed and required (part of #222):
+  `script/bootstrap --cgo` installs `vips-jxl` when its package manager is apk,
+  as Alpine's `vips` package lacks the support, and the runtime stage of the
+  `Dockerfile` installs it too. `imgcache.NewService` fails, naming the fix,
+  when `imageprocessor.CheckJPEGXLSupport` finds that libvips cannot load and
+  save JPEG XL, so pixad does not start without it. A test saves an image as
+  JPEG XL with govips and loads it back. JPEG XL is not yet a format pixa
+  serves.
 - 2026-10-08 SQLite writes no longer fail with "database is locked" under load
   (closes #223): `internal/database` opens the database with one connection, so
   pixa's own reads and writes run on it one at a time instead of competing for