fix: CI cache Debian Live ISO to avoid 1.4GB re-download

Copy the Debian Live ISO from the server's existing build cache
into the CI workspace before running the ISO build. Saves ~10 min.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

.gitea/workflows/build-iso.yml

@@ -23,6 +23,16 @@ jobs:
       - name: Build frontend
         run: cd neode-ui && npm ci && npm run build
 
+      - name: Cache Debian Live ISO
+        run: |
+          WORK_DIR="image-recipe/build/auto-installer"
+          mkdir -p "$WORK_DIR"
+          CACHED="/home/archipelago/archy/image-recipe/build/auto-installer/debian-live-installer.iso"
+          if [ -f "$CACHED" ] && [ ! -f "$WORK_DIR/debian-live-installer.iso" ]; then
+            cp "$CACHED" "$WORK_DIR/debian-live-installer.iso"
+            echo "Cached Debian Live ISO copied ($(du -h "$WORK_DIR/debian-live-installer.iso" | cut -f1))"
+          fi
+
       - name: Build unbundled ISO
         run: |
           cd image-recipe