fix(nextcloud): NULL parent_id before deleting Folder rows on SourceRoot remove

Folders have a self-referential parent_id FK with no ON DELETE rule.
A flat DELETE of the whole subtree trips folders_parent_id_fkey because
postgres checks the constraint per-row regardless of insertion / list
order. Hard-removing 'Taco and Muli - 2024 onward' (35-folder subtree)
returned 500 with ForeignKeyViolationError every attempt.

Fix: UPDATE folders SET parent_id = NULL WHERE id IN (folder_ids) before
the DELETE so the chain is broken cleanly. Same pattern used in
prune_missing_photos for the same constraint.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

backend/app/routers/nextcloud.py

@@ -30,7 +30,7 @@ from typing import Optional
 
 from fastapi import APIRouter, Depends, HTTPException, Query
 from pydantic import BaseModel
-from sqlalchemy import delete, select
+from sqlalchemy import delete, select, update
 from sqlalchemy.ext.asyncio import AsyncSession
 
 from app.database import get_db
@@ -332,6 +332,17 @@ async def delete_nextcloud_source_root(
                 FolderShare.folder_type == 'folder',
             )
         )
+        # Folders have a self-referential parent_id FK with no
+        # ON DELETE rule. A flat DELETE of the whole subtree trips
+        # `folders_parent_id_fkey` because postgres checks the
+        # constraint per row, regardless of insertion / list order.
+        # NULL the parent_id on every folder we're about to delete
+        # first so the chain breaks cleanly.
+        await db.execute(
+            update(Folder)
+            .where(Folder.id.in_(folder_ids))
+            .values(parent_id=None)
+        )
         await db.execute(delete(Folder).where(Folder.id.in_(folder_ids)))
 
     await db.delete(sr)