Custom Chat Persistence

The deliverable is one file in the app — src/lib/chat-persistence.ts — exporting a ChatPersistence built from the database client the app already has. Plus whatever DDL that database needs, added through the app's existing migration flow.

Do not create a package, a second client, or a migration runner.

Route first. If the app already runs one of these, stop and use that skill — it has the driver-specific code: