git: stop tracking the runtime image cache (data/img_cache)

The cache-feature commit swept warmed WebPs into Git via 'git add -A' — 257 tracked
files (~12 MB) + churn making prod checkouts dirty. Add data/img_cache/ to .gitignore
(beside art_cache) and untrack with 'git rm -r --cached' (live files retained; no
history rewrite). [Codex audit blocker]

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

.gitignore

@@ -10,3 +10,4 @@ data/geo_audit*.json
 
 logs/
 data/art_cache/
+data/img_cache/