feat: Add structured template metadata generation with OpenAI

- Implement OpenAI batch API integration for metadata generation
- Add metadata columns to database schema (metadata_json, metadata_generated_at)
- Create MetadataGenerator service with structured output schemas
- Create BatchProcessor for handling OpenAI batch jobs
- Add --generate-metadata flag to fetch-templates script
- Update template repository with metadata management methods
- Add OpenAI configuration to environment variables
- Include comprehensive tests for metadata generation
- Use gpt-4o-mini model with 50% cost savings via batch API

🤖 Generated with [Claude Code](https://claude.ai/code)

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

package.json

@@ -134,8 +134,10 @@
     "n8n": "^1.110.1",
     "n8n-core": "^1.109.0",
     "n8n-workflow": "^1.107.0",
+    "openai": "^4.77.0",
     "sql.js": "^1.13.0",
-    "uuid": "^10.0.0"
+    "uuid": "^10.0.0",
+    "zod": "^3.24.1"
   },
   "optionalDependencies": {
     "@rollup/rollup-darwin-arm64": "^4.50.0",