feat(analytics): first-party /_a footer across generated docs (ADR-0006 Phase 2c) (#44)

#43 updated jsdoc.json (the config), but api-docs deploys by rsyncing the
COMMITTED docs/ build artifact — it does not build on deploy — so the live
site still served the cross-origin tracker. This regenerates the footer in the
committed HTML to match the updated jsdoc.json.

Uniform, surgical swap across all 545 generated pages:
  - src="https://analytics.jellyrock.app/script.js"
  + src="/_a/script.js" … data-host-url="https://api.jellyrock.app/_a"

Identical to what `npm run build` now emits (jsdoc.json is already first-party
on main), so the next code-update regeneration produces no footer diff. Events
POST to /_a/api/send → Caddy strips to umami /api/send. analytics.jellyrock.app
stays live as rollback (Bridge).

Author: cewert