feat(taskbroker): Add Migration Flag and Pipeline Stage

[george-sentry]

Linear

Refs STREAM-1088

Description

We need to perform migrations with a sufficiently privileged user, and we don't want taskbrokers stumbling over each other to perform migrations every time they restart. Instead, we can perform migrations and create all necessary databases during deployment one time.

Changes...

• New --run / -r argument that either runs the broker (default) or migrations
• New pg_ddl_username and pg_ddl_password config fields for the user that will perform migrations
• When run with --run=migrations, it creates the necessary database if it doesn't already exist, runs sqlx::migrate!, and exits

[linear-code]

STREAM-1088

[cursor]

Cursor Bugbot has reviewed your changes and found 2 potential issues.

^{❌ Bugbot Autofix is OFF. To automatically fix reported issues with cloud agents, enable autofix in the Cursor dashboard.}

^{Reviewed by Cursor Bugbot for commit 714044c. Configure here.}