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/build-similarity-cache.yml

@@ -145,12 +145,30 @@ jobs:
           git config --local user.email "github-actions[bot]@users.noreply.github.com"
           git config --local user.name "github-actions[bot]"
           
-          # Switch to or create dedicated cache branch
-          git checkout -b similarity-cache-data || git checkout similarity-cache-data
+          # Fetch all branches
+          git fetch origin
           
-          # Add only the similarity cache files (not all_cards.parquet)
-          git add card_files/similarity_cache.parquet
-          git add card_files/similarity_cache_metadata.json
+          # Try to checkout existing branch, or create new orphan branch
+          if git ls-remote --heads origin similarity-cache-data | grep similarity-cache-data; then
+            echo "Checking out existing similarity-cache-data branch..."
+            git checkout similarity-cache-data
+          else
+            echo "Creating new orphan branch similarity-cache-data..."
+            git checkout --orphan similarity-cache-data
+            git rm -rf . || true
+            # Create minimal README for the branch
+            echo "# Similarity Cache Data" > README.md
+            echo "This branch contains pre-built similarity cache files for the MTG Deckbuilder." >> README.md
+            echo "Updated automatically by GitHub Actions." >> README.md
+          fi
+          
+          # Ensure card_files directory exists
+          mkdir -p card_files
+          
+          # Add only the similarity cache files (use -f to override .gitignore)
+          git add -f card_files/similarity_cache.parquet
+          git add -f card_files/similarity_cache_metadata.json
+          git add README.md 2>/dev/null || true
           
           # Check if there are changes to commit
           if git diff --staged --quiet; then