Add is_default flag for system trees

- Add is_default column to trees table
- Default trees have no author and are visible to all users
- Only admins can create default trees
- Update seed script to mark seeded trees as default
- Update seed script to use CLI auth instead of creating seed user

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

backend/app/api/endpoints/trees.py

@@ -120,13 +120,17 @@ async def create_tree(
     current_user: Annotated[User, Depends(require_engineer_or_admin)]
 ):
     """Create a new tree (engineers and admins only)."""
+    # Only admins can create default/system trees
+    is_default = tree_data.is_default and current_user.role == "admin"
+
     new_tree = Tree(
         name=tree_data.name,
         description=tree_data.description,
         category=tree_data.category,
         tree_structure=tree_data.tree_structure,
-        author_id=current_user.id,
-        team_id=current_user.team_id
+        author_id=None if is_default else current_user.id,  # Default trees have no author
+        team_id=None if is_default else current_user.team_id,
+        is_default=is_default
     )
     db.add(new_tree)
     await db.commit()