Fix YAML parsing error in generated design metadata

Escape double quotes in user-provided name and concept fields to prevent
YAML parsing errors when designs contain quotes (e.g., "tax wealth not work").

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

backend/app/api/design_generator.py

@@ -142,10 +142,13 @@ Square format, clean edges for die-cut sticker."""
     image_path.write_bytes(image_bytes)
 
     # Create metadata.yaml
+    # Escape quotes in user-provided strings to prevent YAML parsing errors
+    safe_name = request.name.replace('"', '\\"')
+    safe_concept = request.concept.replace('"', '\\"')
     tags_str = ", ".join(request.tags) if request.tags else "mycopunk, sticker, ai-generated"
-    metadata_content = f"""name: "{request.name}"
+    metadata_content = f"""name: "{safe_name}"
 slug: {slug}
-description: "{request.concept}"
+description: "{safe_concept}"
 tags: [{tags_str}]
 created: {date.today().isoformat()}
 author: ai-generated