Database backend abstraction (SQLite → PostgreSQL migration path)

[andrewmarconi]

Summary

Abstract the database layer in engine-core to support multiple backends, enabling a future migration from SQLite to PostgreSQL without breaking the application.

Context

The current stack uses better-sqlite3 with Drizzle ORM. The database file lives at ./data/ceegee.db with WAL mode. As the application moves toward production deployment (Docker, multi-instance), PostgreSQL support will be needed.

Scope

• Abstract the database connection layer to support both SQLite and PostgreSQL
• Use Drizzle ORM's multi-dialect support
• Environment-variable-driven backend selection (e.g., DATABASE_URL)
• Ensure migrations work for both backends
• Keep SQLite as the default for local development