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.

pkg/broker/install/sql/migrations/0001_schema.sql

@@ -0,0 +1,6 @@
+-- Dedicated schema for all broker objects.
+CREATE SCHEMA IF NOT EXISTS broker;
+REVOKE ALL ON SCHEMA broker FROM PUBLIC;
+
+-- gen_random_uuid() for lease tokens.
+CREATE EXTENSION IF NOT EXISTS pgcrypto;