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/models/models.go

@@ -16,10 +16,24 @@ type Job struct {
 	RunAs          string    `json:"run_as"`
 	UserLogin      string    `json:"user_login"`
 	ScheduleID     int64     `json:"schedule_id"`
+	TenantID       string    `json:"tenant_id"`
+	AttemptCount   int       `json:"attempt_count"`
+	MaxAttempts    int       `json:"max_attempts"`
+	LeaseToken     string    `json:"lease_token,omitempty"`
+	IdempotencyKey string    `json:"idempotency_key,omitempty"`
 	CreatedAt      time.Time `json:"created_at"`
 	UpdatedAt      time.Time `json:"updated_at"`
 }
 
+// WakeNotification is the payload sent over pg_notify('broker.event', ...).
+// It carries only what a worker needs to decide whether to wake: the queue
+// number. job_id is included solely for logging -- workers always re-claim
+// via broker_get rather than executing the notified id directly.
+type WakeNotification struct {
+	Queue int   `json:"queue"`
+	JobID int64 `json:"job_id,omitempty"`
+}
+
 // Instance represents a broker instance
 type Instance struct {
 	ID          int64     `json:"id"`