IT Assets: add role-based auth system and portal user sync

Auth:
- Fix local login (was redirecting to portal; now authenticates AdminUser directly)
- Portal SSO still takes priority in production via nginx headers

Role system (admin | editor | readonly):
- New app/utils/decorators.py with editor_required and admin_required decorators
- All write routes protected with editor_required (create/edit/delete/import/mask)
- Settings user management protected with admin_required
- Sidebar hides write-only links for readonly users
- Dashboard quick actions and list page buttons hidden for readonly

Settings page:
- Role colour badges (admin=red, editor=blue, readonly=grey)
- Inline role changer per user (dropdown auto-submit)
- Reset password modal per user
- Delete user button with confirmation
- Add user form includes role selector with legend

Portal user sync:
- New /internal/sync-user endpoint receives user pre-creation from portal
- INTERNAL_SYNC_SECRET added to config
- portal/config.py: added internal_url for itassets app so _sync_user_to_app works

IT_asset_management/app/routes/assignments.py

@@ -4,6 +4,7 @@ from flask import (Blueprint, render_template, redirect, url_for,
                    flash, request, current_app)
 from flask_login import login_required, current_user
 from app.extensions import db
+from app.utils.decorators import editor_required
 from app.models.assignment import Assignment
 from app.models.asset import Asset
 from app.models.user import User
@@ -46,6 +47,7 @@ def index():
 
 @bp.route('/new', methods=['GET', 'POST'])
 @login_required
+@editor_required
 def create():
     # Pre-fill from query params (used from asset / user detail pages)
     preselect_asset_id = request.args.get('asset_id', type=int)
@@ -110,6 +112,7 @@ def create():
 
 @bp.route('/<int:assignment_id>/return', methods=['POST'])
 @login_required
+@editor_required
 def return_asset(assignment_id):
     assignment = Assignment.query.get_or_404(assignment_id)