docs(backend): consolidated harvest requirements as one buildable file

HARVEST-BACKEND-REQUIREMENTS.md - single index the backend builds the
fork harvest from, so the FH-* mechanisms are not scattered across nine
phase contracts:

- the nine release invariants (the acceptance gate)
- every FH-* requirement with its exact mechanism and phase-contract
  reference, grouped by area (correctness core, sessions/access,
  tenancy/publish/content, identity, operations)
- 15 acceptance tests mapped to the invariant each guards
- build order, and what the frontend already delivered so the backend
  builds to a known target rather than guessing

Linked from the backend README index.

No implementation changes.

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

docs/backend/README.md

@@ -5,6 +5,8 @@
 > **Implementing tenant routing/nginx? [TENANT-API-DOMAIN-HANDOFF.md](TENANT-API-DOMAIN-HANDOFF.md)** is the final host normalization, CORS, TLS, reverse-proxy, CI-secret, and acceptance contract.
 >
 > **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.
+>
+> **Building from the fork harvest? [HARVEST-BACKEND-REQUIREMENTS.md](HARVEST-BACKEND-REQUIREMENTS.md)** — one consolidated view of every mechanism harvested from the parallel platform (the `FH-*` items): the nine release invariants, each requirement with its mechanism and phase-contract reference, the 15 acceptance tests, and the build order. Start here for the security/correctness hardening; the phase docs hold the full entity shapes.
 
 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.