Send submission and budget events to Quilt #3

Quilt now has a patch API (patchworklabsorg/quilt#8). Krater is the app of the Ganymede patch. Krater must send its submission and budget events to Quilt.

Scope

• Write events to a transactional outbox (quilt_outbox) in the same transaction as the domain change.
• Map Krater's domain to the Quilt event types: submission.created, submission.updated, budget.committed, budget.released, spend.recorded.
• Get a Weave client_credentials token with the quilt scope through krater.weave (feat: let resource servers introspect client_credentials tokens weave#176).
• Send the outbox in a periodic worker task. Keep the order per submission. Retry with backoff. Show permanent failures to admins.
• Add a backfill command for existing projects.
• Document the integration in docs/quilt-integration.md.