unpkg.com fetches were intermittently failing in production, leaving
the /sites-map page without a working map. Vendoring Leaflet (JS, CSS,
marker/layer images) under priv/static/vendor/leaflet/ removes the
runtime CDN dependency, the matching CSP relaxation, and the SRI hash
churn on future updates.
Add 'vendor' to ToweropsWeb.static_paths/0 so Plug.Static serves the
new tree, and revert script-src/style-src to no longer list unpkg.com.
connect-src still allows *.tile.openstreetmap.org for tile fetches.
