fix: clean up folder and tag assignments on tree soft delete

When a tree is soft-deleted, folder assignments and tag assignments are
now removed from junction tables. Tag usage counts are decremented with
a floor of zero to prevent negative counts.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

backend/app/api/endpoints/trees.py

@@ -3,15 +3,15 @@ from typing import Annotated, Optional
 from uuid import UUID
 from fastapi import APIRouter, Depends, HTTPException, status, Query
 from sqlalchemy.ext.asyncio import AsyncSession
-from sqlalchemy import select, func, or_, true as sa_true
+from sqlalchemy import select, func, or_, true as sa_true, update
 from sqlalchemy.orm import selectinload
 
 from app.core.database import get_db
 from app.models.tree import Tree
 from app.models.user import User
 from app.models.category import TreeCategory
-from app.models.tag import TreeTag
-from app.models.folder import UserFolder
+from app.models.tag import TreeTag, tree_tag_assignments
+from app.models.folder import UserFolder, user_folder_trees
 from app.schemas.tree import TreeCreate, TreeUpdate, TreeResponse, TreeListResponse, CategoryInfo
 from app.api.deps import get_current_active_user, require_engineer_or_admin, require_admin
 from app.core.permissions import can_edit_tree, can_access_tree
@@ -522,6 +522,31 @@ async def delete_tree(
     tree.is_active = False
     tree.deleted_at = datetime.now(timezone.utc)
     tree.deleted_by = current_user.id
+
+    # Clean up folder assignments
+    await db.execute(
+        user_folder_trees.delete().where(user_folder_trees.c.tree_id == tree.id)
+    )
+
+    # Decrement usage_count on associated tags (floor at 0)
+    tag_ids_result = await db.execute(
+        select(tree_tag_assignments.c.tag_id).where(
+            tree_tag_assignments.c.tree_id == tree.id
+        )
+    )
+    tag_ids = [row[0] for row in tag_ids_result.fetchall()]
+    if tag_ids:
+        await db.execute(
+            update(TreeTag)
+            .where(TreeTag.id.in_(tag_ids))
+            .values(usage_count=func.greatest(TreeTag.usage_count - 1, 0))
+        )
+
+    # Clean up tag assignments
+    await db.execute(
+        tree_tag_assignments.delete().where(tree_tag_assignments.c.tree_id == tree.id)
+    )
+
     await log_audit(db, current_user.id, "tree.delete", "tree", tree.id,
                     {"tree_name": tree.name})
     await db.commit()