chore(cms): remove Strapi provider and rollback path

[keshav-epyc]

Both staging and production have run on CMS_PROVIDER=payload since the cutover (#111, #112). The Strapi provider was dead code that still shipped in the Worker bundle and still had STRAPI_* secrets pushed on every deploy.

What this removes

• lib/strapi/{client,types}.ts and lib/cms/strapi-provider.ts
• The provider switch itself — getCMS() now constructs PayloadProvider unconditionally. getCMSProviderName / CMSProviderName / the CMS_PROVIDER var are gone from lib/cms/config.ts, wrangler.jsonc (all three blocks), both deploy workflows, and .env.example.
• scripts/cms-parity.ts and the cms:parity package script — it existed to diff the two providers.
• STRAPI_URL / STRAPI_API_TOKEN / STRAPI_PREVIEW from both workflows' build env and their wrangler secret bulk payloads, and from cloudflare-env.d.ts.

Bugs fixed along the way

The default CMS_PROVIDER in wrangler.jsonc was still "strapi", and getCMSProviderName() fell back to 'strapi' on any unknown value. A fresh checkout with no .env therefore read Strapi in local dev. That failure mode is gone with the variable.

The default PAYLOAD_URL also still pointed at cms.epyc.in — the old Strapi host. Now epyc-payload-cms.epyc.workers.dev, matching what both named environments already override it to.

Kept deliberately

Comments mentioning legacyStrapiId in payload-provider.ts stay — that field exists in Payload and drives sort order for blogs, projects, and gallery. Removing the explanation would make those sort strings look arbitrary.

Verification

• pnpm exec tsc --noEmit — clean
• pnpm test — 114/114 pass
• pnpm exec eslint — 0 errors (6 pre-existing warnings, none in changed files)

No runtime behaviour change: every environment already resolved to PayloadProvider.

Follow-ups after merge (not in this diff)

1. wrangler secret bulk upserts and never deletes, so the Strapi secrets still sit on both Workers:

   for e in staging production; do for s in STRAPI_URL STRAPI_API_TOKEN STRAPI_PREVIEW; do
     pnpm exec wrangler secret delete $s --env $e; done; done
   

2. STAGING_STRAPI_* / PRODUCTION_STRAPI_* GitHub Actions secrets are now unreferenced — remove in repo settings.
3. docs/payload-*.md are left as historical migration records, including a rollback section that no longer applies.

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