feat(images): multi-format portfolio pipeline (AVIF/WebP/JPEG/JXL)

Backend now stores AVIF, WebP, JPEG, and optional JXL variants for both full-size and thumbnail images. Database schema extended with 7 new columns. Image validation supports AVIF and JXL magic bytes. Comprehensive test coverage for all format URL fields and magic byte detection. Legacy single-URL images remain backward-compatible.

Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-openagent)

Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>

init-scripts/init-script.sql

@@ -576,7 +576,14 @@ create table images (
     url text not null,
     thumbnail_url text not null,
     tag_names text[] not null default '{}',
-    created_at timestamptz not null default now()
+    created_at timestamptz not null default now(),
+    full_avif_url text,
+    full_webp_url text,
+    full_jpg_url text,
+    full_jxl_url text,
+    thumb_avif_url text,
+    thumb_webp_url text,
+    thumb_jpg_url text
 );
 
 -- only for autocomplete text