feat(affiliate): admin CRUD — routes, list/form templates, sidebar entry
Routes: GET/POST affiliate, affiliate/results (HTMX), affiliate/new,
affiliate/<id>/edit, affiliate/<id>/delete, affiliate/<id>/toggle.
Templates: affiliate_products.html (filterable list), affiliate_form.html
(two-column with live preview slot), partials/affiliate_row.html,
partials/affiliate_results.html. Affiliate added to base_admin.html sidebar
and subnav (Products | Dashboard).
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
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