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>

.env.example

@@ -86,4 +86,24 @@ AUTH_TOKEN=your-secure-token-here
 # N8N_API_TIMEOUT=30000
 
 # Maximum number of API request retries (default: 3)
-# N8N_API_MAX_RETRIES=3
+# N8N_API_MAX_RETRIES=3
+
+# =========================
+# OPENAI API CONFIGURATION
+# =========================
+# Optional: Enable AI-powered template metadata generation
+# Provides structured metadata for improved template discovery
+
+# OpenAI API Key (get from https://platform.openai.com/api-keys)
+# OPENAI_API_KEY=
+
+# OpenAI Model for metadata generation (default: gpt-4o-mini)
+# OPENAI_MODEL=gpt-4o-mini
+
+# Batch size for metadata generation (default: 100)
+# Templates are processed in batches using OpenAI's Batch API for 50% cost savings
+# OPENAI_BATCH_SIZE=100
+
+# Enable metadata generation during template fetch (default: false)
+# Set to true to automatically generate metadata when running fetch:templates
+# METADATA_GENERATION_ENABLED=false