Refactor V1 tenant routes

[haocn-ops]

Summary

• split V1 tenant route handling into mailbox, message, and webhook route modules
• extract shared V1 authz, metering, validation, and response helpers
• reduce src/v1/index.js to top-level route aggregation and add focused helper coverage

Testing

• node --test test/v1-helpers.test.js test/fetch-app.test.js test/api.test.js
• npm test