cli: enable embedded-postgres via config instead of env var

Rather than requiring the `EXPERIMENTAL_DEV_DB` environment variable,
the embedded postgres server is now started automatically when
`backend.database.client` is set to `embedded-postgres` in the app
config. The CLI reads the config before spawning the backend and
injects the actual pg connection details via env override.

Signed-off-by: Patrik Oldsberg <poldsberg@gmail.com>
Made-with: Cursor

packages/backend-defaults/config.d.ts

@@ -585,7 +585,7 @@ export interface Config {
     /** Database connection configuration, select base database type using the `client` field */
     database: {
       /** Default database client to use */
-      client: 'better-sqlite3' | 'sqlite3' | 'pg';
+      client: 'better-sqlite3' | 'sqlite3' | 'pg' | 'embedded-postgres';
       /**
        * Base database connection string, or object with individual connection properties
        * @visibility secret