/api/slide-rank — read + write per-slide classifier state into the consumer's audits file

Dev-only API route (`prerender = false`) that reads + writes the audit registry at the consumer's `data/audits/slides.json` (default path; configurable via the integration options). GET returns the full ranks map; POST upserts a single `{deckSlug, variantSlug, slot, status}` entry with `rankedAt` ISO timestamp + `rankedBy: 'founder'`. Status `'pending'` is the implicit default — never persisted; a POST with `status === 'pending'` deletes the existing entry. Fails-soft in production static builds (the route doesn't exist there, both GET and POST 404 — SlideRankPill catches and degrades to read-only).