feat: optimize cache workflow with orphan branch and age check

- Create/use orphan branch 'similarity-cache-data' for cache distribution
- Add age check to dockerhub-publish: only rebuild if cache >7 days old
- Use git add -f to force-add cache files (keeps .gitignore clean)
- Weekly scheduled builds will keep cache fresh automatically

This avoids rebuilding cache on every Docker publish while ensuring
cache is always reasonably fresh (<7 days old).

.github/workflows/dockerhub-publish.yml

@@ -7,15 +7,53 @@ on:
   workflow_dispatch:
 
 jobs:
+  check-cache-age:
+    name: Check similarity cache age
+    runs-on: ubuntu-latest
+    outputs:
+      needs_rebuild: ${{ steps.check.outputs.needs_rebuild }}
+    steps:
+      - name: Check cache age
+        id: check
+        run: |
+          # Check if cache is older than 7 days
+          CACHE_URL="https://raw.githubusercontent.com/${{ github.repository }}/similarity-cache-data/card_files/similarity_cache_metadata.json"
+          
+          if wget -q --spider "$CACHE_URL"; then
+            wget -q "$CACHE_URL" -O metadata.json
+            BUILD_DATE=$(jq -r '.build_date' metadata.json)
+            
+            # Calculate age in seconds
+            BUILD_EPOCH=$(date -d "$BUILD_DATE" +%s 2>/dev/null || echo 0)
+            NOW_EPOCH=$(date +%s)
+            AGE_DAYS=$(( ($NOW_EPOCH - $BUILD_EPOCH) / 86400 ))
+            
+            echo "Cache age: $AGE_DAYS days"
+            
+            if [ $AGE_DAYS -gt 7 ]; then
+              echo "needs_rebuild=true" >> $GITHUB_OUTPUT
+              echo "Cache is stale (>7 days), will rebuild"
+            else
+              echo "needs_rebuild=false" >> $GITHUB_OUTPUT
+              echo "Cache is fresh (<7 days), skipping rebuild"
+            fi
+          else
+            echo "needs_rebuild=true" >> $GITHUB_OUTPUT
+            echo "Cache not found, will build"
+          fi
+
   build-cache:
     name: Build similarity cache
+    needs: check-cache-age
+    if: needs.check-cache-age.outputs.needs_rebuild == 'true'
     uses: ./.github/workflows/build-similarity-cache.yml
     secrets: inherit
 
   prepare:
     name: Prepare metadata
     runs-on: ubuntu-latest
-    needs: build-cache
+    needs: [check-cache-age, build-cache]
+    if: always() && (needs.build-cache.result == 'success' || needs.build-cache.result == 'skipped')
     permissions:
       contents: read
     outputs: