docs: final backend handoff - revision endpoints, response-shape additions, close-out

Updates FRONTEND-API-SURFACE-COMPLETE.md to cover everything built after it
was first generated - this is the final version for this pass, not a
snapshot mid-way through.

Added, none of which existed in the doc before:

- §18: the 4 marketplace revision endpoints (create draft/validate/
  publish/rollback) - built this session, missing from the census entirely.
  Flags the same draft/validated/preview/published ambiguity already
  documented in the model itself, so backend sees it without having to
  read source.
- §19: RoutingContext as an optional field addition on GET
  /api/admin/v2/orders/{id} - not a new endpoint, a response-shape ask.
- §20: order pricing-breakdown fields (unitPriceMinor, lineTotalMinor,
  priceSnapshotId, discountMinor, fxQuoteId, deliveryMinor) feeding the new
  total-formula panel. States plainly that the panel shows nothing rather
  than a wrong number while these are absent.
- §21: 8 optional dashboard-metrics fields (GMV, conversion, moderation
  queue, etc.) - explicitly labelled a genuine ask, not a confirmed
  contract, since no spec exists for this endpoint at all (§15).
- §22: states directly that the frontend's new double-click guard does not
  replace backend idempotency enforcement and was never meant to - closes
  one UI race, does nothing for a retried request or a duplicate webhook.

Counts updated (47->51 specified, 86->90 total; +3 response-shape asks that
aren't new endpoints). Closing section states the one item genuinely
blocked pending a live backend (F60, the full acceptance-path E2E) so
nobody mistakes "frontend backlog complete" for "nothing left to build."

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

docs/backend/README.md

@@ -2,7 +2,7 @@
 
 > **New here? Start with [BACKEND-HANDOFF.md](BACKEND-HANDOFF.md)** — reading order, current infrastructure state, auth surface, and what a working dev environment still needs.
 >
-> **Want every endpoint in one place? [FRONTEND-API-SURFACE-COMPLETE.md](FRONTEND-API-SURFACE-COMPLETE.md)** — generated directly from source, all 86 endpoints the frontend currently calls, marked Specified / Inferred / Undocumented against the contracts below. Use it to see gaps across all contracts at once; use the individual Phase/Track docs for full entity shapes and invariants.
+> **Want every endpoint in one place? [FRONTEND-API-SURFACE-COMPLETE.md](FRONTEND-API-SURFACE-COMPLETE.md)** — the final handoff doc. Generated directly from source, all 90 endpoints the frontend currently calls plus 3 response-shape additions on existing endpoints, marked Specified / Inferred / Undocumented against the contracts below. Use it to see gaps across all contracts at once; use the individual Phase/Track docs for full entity shapes and invariants.
 
 This directory is the complete set of wire contracts for building the backend behind [Product Plan v3.1](../PRODUCT-PLAN-v3.1-GAP-ANALYSIS.md). Each doc specifies entities, endpoints, and invariants only — never DB schema or service boundaries, which stay backend's own call.