build(monitor): pin the D1 database id for auth-monitor

[matthew-demidoff]

Summary

• pin the auth-monitor D1 database id in monitor/wrangler.jsonc so deploys and migrations target the same database
• document that the database is created once with wrangler d1 create before the first deploy, and that a non-interactive shell does not write the id into the config

Verification

• monitor typecheck and tests pass locally
• deployed with this config: wrangler deploy resolves env.DB to auth-monitor, migrations applied, heartbeat pings from the worker and bot land in the heartbeats table, all four checks report up