Art post-audit polish (Codex): image HEAD, texture immutable cache, lightbox a11y, spacing

- /api/art/image/{id} now answers HEAD as well as GET (was 404 on HEAD) — mirrors the
  /a/{id} fix. Added tests/test_art_api.py (GET+HEAD+size=full fallback + today payload).
- /textures/* served immutable (long cache) instead of no-cache; excluded from the
  revalidate matcher. Live Caddyfile + repo snapshot both updated.
- Lightbox: Escape closes it, and focus moves to it on open (keyboard-friendly).
- Trimmed the gallery's top padding so "Daily Art" sits closer to the bar.

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

goodnews/api.py

@@ -2279,7 +2279,7 @@ def create_app() -> FastAPI:
             "image_url_large": f"/api/art/image/{a['object_id']}?size=full",
         }
 
-    @app.get("/api/art/image/{object_id}")
+    @app.api_route("/api/art/image/{object_id}", methods=["GET", "HEAD"])
     def art_image(object_id: int, size: str = Query("")) -> FileResponse:
         cdir = art.cache_dir()
         matches = sorted(cdir.glob(f"{object_id}-full.*")) if size == "full" else []