Add post/page identity synchronization to Netlify

[aileen]

Parent

• PRD #213 — Deliver the maintained HTML-to-Algolia extraction pipeline
• User stories covered: 37–40, 43, 48–52

What to build

Complete incremental post/page synchronization after the CLI identity migration release exists. Move existing post replacement/deletion to Ghost content identity, add parallel page handlers, document the exact Ghost event wiring, preserve current public post contracts, and publish the Netlify release that delivers page indexing end to end.

Acceptance criteria

• Existing post-published and post-unpublished exports, basenames, envelopes, activation/authentication behavior, and valid posts-only outcomes remain compatible.
• Post replacement and deletion operate by stable Ghost content ID rather than slug.
• New page-published and page-unpublished public handlers mirror the post handler contract with type-specific page envelopes.
• Documentation maps post.published, post.published.edited, post.unpublished, post.deleted, page.published, page.published.edited, page.unpublished, and page.deleted to the four exact handler basenames; generic edited events are not registered.
• Publish/edit handlers use the complete type-specific current resource directly, validate/transform/size-check/preflight before mutation, and replace all records for the content ID without a Content API re-fetch.
• Unpublish/delete handlers delete every record for the content ID from current, falling back to previous when required.
• No handler reads or writes Algolia settings.
• Native handler acceptance covers all four public basenames, post compatibility, page envelopes, current/previous fallback, same-slug independence, shrinking replacement, deletion, malformed bodies, external failures, and no re-fetch/settings calls.
• The Netlify minor is released only after the CLI migration release exists; deployment documentation requires a successful complete migration before writes resume.
• Release notes list four handlers, eight events, required migration/settings, deployment order, compatible package floors, verification, and index/package rollback.
• Issue Cannot index pages instead of posts #4 is closed only after npm and packed/native handler verification prove posts-only compatibility plus page synchronization, with exact delivered versions recorded.

Blocked by

• #225 — Add CLI Ghost content type selection and guarded migration