feat: Implement contact submission archiving and deletion

supabase/migrations/20251028193251_0c37046b-3d85-4d8a-a6a4-b24d79ce5b3b.sql

@@ -0,0 +1,31 @@
+-- Add archive support columns to contact_submissions
+ALTER TABLE contact_submissions
+  ADD COLUMN archived_at timestamp with time zone,
+  ADD COLUMN archived_by uuid REFERENCES auth.users(id);
+
+-- Add comments for clarity
+COMMENT ON COLUMN contact_submissions.archived_at IS 'Timestamp when submission was archived (soft deleted)';
+COMMENT ON COLUMN contact_submissions.archived_by IS 'User who archived this submission';
+
+-- Index for performance on archived queries
+CREATE INDEX idx_contact_submissions_archived ON contact_submissions(archived_at) 
+  WHERE archived_at IS NOT NULL;
+
+-- Add FK constraint to ensure email threads are deleted with submission (cascade)
+ALTER TABLE contact_email_threads
+  ADD CONSTRAINT fk_submission_cascade
+  FOREIGN KEY (submission_id)
+  REFERENCES contact_submissions(id)
+  ON DELETE CASCADE;
+
+-- Add comment on constraint
+COMMENT ON CONSTRAINT fk_submission_cascade ON contact_email_threads IS 
+  'Cascade delete email threads when parent submission is deleted';
+
+-- Allow moderators to DELETE contact submissions (hard delete with MFA)
+CREATE POLICY "Moderators can delete contact submissions" ON contact_submissions
+  FOR DELETE
+  TO authenticated
+  USING (
+    is_moderator(auth.uid()) AND has_aal2()
+  );