Centralises retailer config in affiliate_programs table (URL template,
tracking tag, commission %). Products now use program dropdown + product
identifier instead of manual URL baking. URL assembled at redirect time
via build_affiliate_url() — changing a tag propagates to all products
instantly. Backward compatible: legacy baked-URL products fall through
unchanged. Amazon OneLink (configured in Associates dashboard) handles
geo-redirect to local marketplaces with no additional programs needed.
Also fixes _rebuild_article() frontmatter rendering bug.
Commits: fix frontmatter, migration 0027, program CRUD functions,
redirect update, admin CRUD + templates, product form update, tests.
41 tests, all passing. Ruff clean.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>