feat(delivery): add automatic rollback state machine for failed deployments

Introduce a compensating workflow that launches the dedicated AWX
rollback template when a deployment cannot be started or fails midway.

- add task states: rollback_pending, rolling_back, rolled_back,
  rollback_failed, rollback_acknowledged
- add RollbackJob model to track the compensating AWX run separately
  from the deploy run, preserving both job IDs for audit
- hold resource reservations in 'rollback' status until cleanup
  succeeds so a failed cleanup cannot be masked by a later delivery
- poll rollback jobs with a 2-minute launch timeout; an unknown launch
  result surfaces as a recoverable failure instead of re-launching
- protect finished/register_failed/rolled-back tasks from rollback;
  register_failed keeps the healthy instance and its resource usage
- add DELIVERY_ROLLBACK_TEMPLATE_ID config; without it, failures are
  marked rollback_failed and require manual cleanup
- use unique pending-<task_id> placeholder for executor job IDs

server/internal/config/config.go

@@ -63,6 +63,7 @@ type Config struct {
 	AWXToken                  string
 	AWXUsername               string
 	AWXPassword               string
+	RollbackTemplateID        uint64
 	DeliveryServiceToken      string
 	DeliverySchedulerEnabled  bool
 	DeliveryPollSeconds       int
@@ -133,6 +134,7 @@ func Load() Config {
 		AWXToken:                  env("AWX_TOKEN", ""),
 		AWXUsername:               env("AWX_USERNAME", ""),
 		AWXPassword:               env("AWX_PASSWORD", ""),
+		RollbackTemplateID:        uint64(envInt("DELIVERY_ROLLBACK_TEMPLATE_ID", 0)),
 		DeliveryServiceToken:      env("DELIVERY_SERVICE_TOKEN", ""),
 		DeliverySchedulerEnabled:  envBool("DELIVERY_SCHEDULER_ENABLED", false),
 		DeliveryPollSeconds:       envInt("DELIVERY_POLL_SECONDS", 5),