docs: master API surface list, all 86 frontend endpoints in one doc

Generated directly from source (every this.http.get/post/patch/put/delete
call across core/, features/admin/, api.service.ts) rather than written
from memory - a census, not a design doc.

47 already match an existing Phase/Track contract exactly. 24 are inferred
from this codebase's own REST conventions with no contract doc stating them
- each flagged in source at its call site, not just in this doc, so backend
sees the reasoning next to the code. 15 are legacy endpoints
(/category, /cart, /qr, /websession, ...) with no contract anywhere,
still live today.

Biggest concrete gap surfaced: three full admin domains (transactions,
monitoring, moderation) have real UI and real gateways calling
/api/admin/v2/{resource} by convention, with zero backend contract written
for any of them.

One real bug found and fixed while building this, not just flagged:
connector-api.gateway.ts's replay() called
POST /api/admin/v2/integrations/dead-letter/{id}/replay, omitting the
{connectorId} segment the contract's own path requires
(PHASE-4-CONNECTOR-FRAMEWORK-CONTRACT.md §7). Fixed the interface, both
gateway implementations, and the doc entry in the same pass - no callers
existed yet, so this shipped without ever being exercised by a UI.

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

docs/backend/README.md

@@ -1,6 +1,8 @@
 # Backend Contracts Index — Product Plan v3.1
 
 > **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.
 
 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.