feat: cron arithmetic and persisted workflow schedules

server/internal/models/workflow.go

@@ -44,6 +44,20 @@ type StepOverride struct {
 	SecretRefs []string `bson:"secret_refs,omitempty" json:"secret_refs,omitempty"`
 }
 
+type Schedule struct {
+	Enabled bool   `bson:"enabled" json:"enabled"`
+	Cron    string `bson:"cron" json:"cron"` // 5-field: minute hour dom month dow
+	TZ      string `bson:"tz" json:"tz"`     // IANA name, e.g. Europe/London
+}
+
+// Skip records why an occurrence did not run. Recording a reason nobody reads
+// is the same as not recording one, so this is surfaced in the UI.
+type Skip struct {
+	Reason string    `bson:"reason" json:"reason"` // "missed" | "already_running"
+	Due    time.Time `bson:"due" json:"due"`
+	At     time.Time `bson:"at" json:"at"`
+}
+
 type Workflow struct {
 	ID              bson.ObjectID     `bson:"_id,omitempty" json:"-"`
 	InstanceID      string            `bson:"instance_id" json:"instance_id"`
@@ -52,6 +66,10 @@ type Workflow struct {
 	TargetServerIDs []string          `bson:"target_server_ids" json:"target_server_ids"`
 	TargetTags      map[string]string `bson:"target_tags,omitempty" json:"target_tags,omitempty"`
 	Steps           []WorkflowStepRef `bson:"steps" json:"steps"`
+	Schedule        *Schedule         `bson:"schedule,omitempty" json:"schedule,omitempty"`
+	NextRunAt       *time.Time        `bson:"next_run_at,omitempty" json:"next_run_at,omitempty"`
+	LastRunAt       *time.Time        `bson:"last_run_at,omitempty" json:"last_run_at,omitempty"`
+	LastSkipped     *Skip             `bson:"last_skipped,omitempty" json:"last_skipped,omitempty"`
 	CreatedAt       time.Time         `bson:"created_at" json:"created_at"`
 	UpdatedAt       time.Time         `bson:"updated_at" json:"updated_at"`
 }