fix: deleted mails showing in inbox — set deleted_at on move to Trash, show in Trash folder

- delete_mail: set deleted_at when moving to Trash (not just folder_id)
- empty_folder: same — set deleted_at when moving to Trash
- list_mails: show soft-deleted mails when viewing Trash folder
- This prevents sync from moving deleted mails back to inbox
- Trash folder shows deleted_at mails, all other folders hide them

app/plugins/builtins/mail/routes.py

@@ -669,6 +669,7 @@ async def empty_folder(
         else:
             # Move to Trash folder (keep visible in Trash)
             mail.folder_id = trash_folder.id
+            mail.deleted_at = now
             try:
                 await mail_services.imap_delete_mail(db, mail.id, tenant_id, permanent=False)
             except Exception as exc:
@@ -1648,7 +1649,11 @@ async def delete_mail(
             await db.flush()
     elif trash_folder:
         # Mail is NOT in Trash → move to Trash folder
+        # Set deleted_at so the mail disappears from the source folder
+        # and the sync doesn't move it back. list_mails shows deleted_at
+        # mails when viewing the Trash folder.
         mail.folder_id = trash_folder.id
+        mail.deleted_at = datetime.now(UTC)
         await db.flush()
         try:
             await mail_services.imap_delete_mail(db, m_id, tenant_id, permanent=False)
@@ -1757,11 +1762,28 @@ async def list_mails(
     if folder_id:
         f_id = _parse_uuid(folder_id, "folder_id")
         stmt = stmt.where(Mail.folder_id == f_id)
+        # Check if this folder is a Trash folder — if so, show soft-deleted mails too
+        folder = (
+            await db.execute(
+                select(MailFolder).where(
+                    and_(MailFolder.id == f_id, MailFolder.tenant_id == tenant_id)
+                )
+            )
+        ).scalar_one_or_none()
+        is_trash = folder and (
+            'trash' in (folder.imap_name or '').lower()
+            or 'papierkorb' in (folder.name or '').lower()
+            or 'trash' in (folder.name or '').lower()
+        )
+        if not is_trash:
+            # Exclude soft-deleted mails in non-Trash folders
+            stmt = stmt.where(Mail.deleted_at.is_(None))
+    else:
+        # No folder specified — exclude soft-deleted mails
+        stmt = stmt.where(Mail.deleted_at.is_(None))
     if account_id:
         a_id = _parse_uuid(account_id, "account_id")
         stmt = stmt.where(Mail.account_id == a_id)
-    # Exclude soft-deleted mails
-    stmt = stmt.where(Mail.deleted_at.is_(None))
     total = (await db.execute(select(func.count()).select_from(stmt.subquery()))).scalar()
     # Dynamic sorting
     sort_columns = {