[Editorial taxonomy] Add cross-category series and geographic tags after rubric rollout #112

Context

Editorial rubric rollout: PR #111. Six main sections are not the same as reusable editorial series. Africa must be a geography filter, not an all-purpose category.

Four editorial series (FR / EN)

1. Sous le capot / Under the Hood — investigated deep dives twice a week on real Cameroonian tools and apps (including anglophone ecosystem); verify claims, tariffs, competing products and limitations.
2. Le Cameroun face au monde / Africa vs the World — dated international comparisons.
3. On a testé / Tried & Tested — actual hands-on reviews with reproducible methods.
4. Campus sans frontières / Beyond Campus — scholarships, admissions and IT careers, with verified deadlines.

Tasks

• Define a structured editorial series field in Payload for posts (zero or more), FR/EN labels, archives and canonical/hreflang. Preserve one main rubric.
• Add geographic tags (Cameroon, anglophone regions, Nigeria, Ghana, Kenya, Rwanda, Uganda, South Africa, Europe and Global), separate from rubrics.
• Distinguish EN-original editorial topics from adapted/translated equivalents and track target SEO intent.
• Add series cards and archive links only when content exists.
• Classify existing stories via editor-reviewed action; never silently rewrite published articles.
• Reconcile recently merged GEO PR GEO: make Trigenys Insights discoverable and citable by AI answer engines #110 Payload migrations before adding new schema migrations.
• Validate that tags do not create duplicate indexed URLs; add sitemap and link regression tests.

Safety

Outside PR #111: DB schema changes, published content reclassification, scraping, automatic publication.