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.

.env.example

@@ -0,0 +1,11 @@
+# Copy to .env (gitignored) and fill in real values.
+# POSTGRES_PASSWORD is the postgres superuser password (used only by the
+# one-shot `migrate` service to run `install --with-roles`).
+# The three BROKER_*_PASSWORD values must match the passwords you put in
+# broker.docker.yaml (copied from broker.docker.example.yaml) for the
+# corresponding role -- BROKER_RUNTIME_PASSWORD in particular must match
+# the `password:` field the broker service itself connects with.
+POSTGRES_PASSWORD=change_me
+BROKER_ADMIN_PASSWORD=change_me
+BROKER_RUNTIME_PASSWORD=change_me
+BROKER_ENQUEUE_PASSWORD=change_me