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/model/delivery.go

@@ -14,6 +14,11 @@ const (
 	TaskValidationFailed = "validation_failed"
 	TaskCanceling        = "canceling"
 	TaskCanceled         = "canceled"
+	TaskRollbackPending  = "rollback_pending"
+	TaskRollingBack      = "rolling_back"
+	TaskRolledBack       = "rolled_back"
+	TaskRollbackFailed   = "rollback_failed"
+	TaskRollbackAck      = "rollback_acknowledged"
 )
 
 type ResourceQuota struct {
@@ -110,6 +115,20 @@ type ExecutionJob struct {
 	UpdatedAt      time.Time  `json:"updated_at"`
 }
 
+// RollbackJob tracks the compensating AWX run independently from the deploy run.
+// Keeping a separate record preserves both job IDs for audit and retry tooling.
+type RollbackJob struct {
+	ID            uint64     `gorm:"primaryKey" json:"id"`
+	TaskID        string     `gorm:"size:36;not null;uniqueIndex" json:"task_id"`
+	ExecutorJobID string     `gorm:"size:128;not null" json:"executor_job_id"`
+	Status        string     `gorm:"size:32;not null;index" json:"status"`
+	Reason        string     `gorm:"type:text" json:"reason"`
+	StartedAt     *time.Time `json:"started_at,omitempty"`
+	FinishedAt    *time.Time `json:"finished_at,omitempty"`
+	CreatedAt     time.Time  `json:"created_at"`
+	UpdatedAt     time.Time  `json:"updated_at"`
+}
+
 type TaskEvent struct {
 	ID        uint64    `gorm:"primaryKey" json:"id"`
 	TaskID    string    `gorm:"size:36;not null;index" json:"task_id"`