- Add _sanitize_banner_html() to src/setup_banner.py: strips <script>/
<iframe> blocks, on* event-handler attributes, and javascript:/data:
URI schemes post-render (I-2). Defense-in-depth — /setup is partly
anonymous so malformed admin content must not execute in visitors'
browsers.
- Apply sanitizer in render_setup_banner() before returning rendered HTML.
- Add 3 unit tests: test_render_strips_script_tags,
test_render_strips_event_handlers, test_render_strips_javascript_uri.
- Drop unused Optional import from src/repositories/welcome_template.py
and src/repositories/setup_banner.py (M-6).
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