feat: align controlled AI sessions and MVP controls

backend/internal/models/ai_session.go

@@ -11,6 +11,7 @@ type SenlinAgentAISession struct {
 	CreatedByIdentity string `gorm:"type:char(36);index"`
 	Title             string `gorm:"not null"`
 	Context           string `gorm:"type:text"`
+	Status            string `gorm:"not null;default:ready"`
 	CreatedAt         time.Time
 	UpdatedAt         time.Time
 }