Metered API Marketplace

Provide a production-lean template for: OpenClaw Skill → Public API Endpoint → Usage Metering → Crypto Payment Gateway → BTC/ETH wallets.

This skill ships a runnable reference server (Fastify + SQLite) that:

• Accepts structured JSON input
• Performs a high-value transformation (pluggable “transformers”)
• Returns structured JSON output
• Enforces signed API key auth
• Checks prepaid balance, deducts per call, and logs usage
• Accepts payment webhooks (Coinbase Commerce / BTCPay Server style)
• Applies a 2.5% platform fee in the ledger (fee addresses configurable)

Workflow (do this in order)