Add approved_at column and update flow

Implements migration to add approved_at to submission_items, creates index, and updates process_approval_transaction to set approved_at on approvals. Also updates TypeScript types to include approved_at and aligns edge function behavior accordingly.

src/types/submissions.ts

@@ -43,6 +43,7 @@ export interface SubmissionItemData {
   rejection_reason: string | null;
   created_at: string;
   updated_at: string;
+  approved_at: string | null;
 }
 
 export interface EntityPhotoGalleryProps {