docs(backend): consolidate all backend contracts into one file

Collapses the entire docs/backend/ set - Phase 1-10, Track A/S, the
partner API, the two handoffs, the frontend surface inventory, and the
harvest requirements - into a single source of truth,
docs/backend/BACKEND-INTEGRATION.md.

Every contract's entities, endpoints, and invariants are preserved,
reorganised by domain rather than by sprint. The nine release
invariants, the FH-* harvest mechanisms, the RBAC/audit/secrets
cross-cutting rules, the tenant-routing infra contract, the 15
acceptance tests, build order, dev setup, and open decisions are all in
the one file, with a change log (§14) at the bottom.

The file opens with the maintenance rule: any new backend need, contract
change, or shipped item updates this file in the same change - the
affected section and the change log. No new backend .md files.

Inbound links from BACKEND-API-REFERENCE, the ADRs, the fork docs,
DEPLOYMENT, PACKAGES-USAGE, the delivery plan, and e2e/README are
repointed at the single doc (section anchors collapse to the file; the
prose section refs remain as context). Also recorded the rule in the
repo CLAUDE.md.

17 backend docs removed, 1 added. No implementation changes.

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

docs/DEPLOYMENT.md

@@ -5,7 +5,7 @@ separate developer's responsibility. API hostnames are separate reverse proxies
 and return `502` until their configured upstream exists (production currently
 defaults to `https://127.0.0.1:445`).
 
-**Multi-tenant, one bundle.** Every customer domain is served by the same build. The SPA resolves its tenant from the `Host` header ([BACKEND-HANDOFF §1a](backend/BACKEND-HANDOFF.md)). One deploy updates every domain simultaneously — there is no per-tenant build and no per-tenant deploy.
+**Multi-tenant, one bundle.** Every customer domain is served by the same build. The SPA resolves its tenant from the `Host` header ([BACKEND-HANDOFF §1a](backend/BACKEND-INTEGRATION.md)). One deploy updates every domain simultaneously — there is no per-tenant build and no per-tenant deploy.
 
 The SPA derives one API origin from the storefront's base domain:
 `example.com`, `store1.example.com`, and `www.example.com` all use
@@ -148,7 +148,7 @@ sudo bash setup-wildcard-tls.sh \
 
 Wildcards require DNS-01 validation, so certbot must write a `_acme-challenge` TXT record. With a provider plugin (`cloudflare`, `route53`) renewal is unattended. `--dns manual` works but prompts for a TXT record at **every** renewal — fine to prove the setup out, not acceptable as a steady state.
 
-**Hostinger has no certbot plugin.** If DNS lives there: either move DNS to a provider that has one (Cloudflare is free, minutes of work), or drive issuance from the [Phase 9](backend/PHASE-9-TENANT-REGISTRY-DOMAINS-CONTRACT.md) domain-automation API once it exists.
+**Hostinger has no certbot plugin.** If DNS lives there: either move DNS to a provider that has one (Cloudflare is free, minutes of work), or drive issuance from the [Phase 9](backend/BACKEND-INTEGRATION.md) domain-automation API once it exists.
 
 ### 4.2 Reconciler — tenants on their own domains