feat(broker): migrations-based install, roles, RLS, and job dependency groups

Replace the ad-hoc tables/procedures install layout with versioned,
ordered SQL migrations tracked in broker_schema_migrations. Add
optional least-privilege role provisioning (--with-roles), multi-tenant
row-level security, lease-based job claiming with stale-lease recovery,
and job dependencies -- both by job id and by fan-in job group. Add
Docker/Compose support for running the broker and its test suite.

.gitignore

@@ -32,7 +32,9 @@ bin/
 broker.yaml
 broker.yml
 broker.json
+broker.docker.yaml
 !broker.example.yaml
+!broker.docker.example.yaml
 
 # IDE
 .vscode/