docs: backend handoff, package usage guide, finalized CI/CD

- docs/backend/BACKEND-HANDOFF.md: single entry point for a backend dev -
  reading order, verified infrastructure state (nginx running, Postgres
  inactive, no API on :8080, no TLS, no DNS automation, no CI runner),
  auth surface, and the day-one setup that is still outstanding
- docs/PACKAGES-USAGE.md: install, required DI providers, full exported
  API for both auth mechanisms, and how to ship a package change
- PACKAGE-EXTRACTION.md now covers build/release/infra only and points at
  the usage guide; CI section reflects the two real workflows

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

docs/backend/README.md

@@ -1,5 +1,7 @@
 # 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.
+
 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.
 
 **Read order matches build order.** Every doc after Phase 1 depends on the ones before it (noted at the top of each). All Sprint 0.1 decisions referenced throughout were answered 2026-08-17 — see [PRODUCT-PLAN-v3.1-DELIVERY-PLAN.md](../PRODUCT-PLAN-v3.1-DELIVERY-PLAN.md) Sprint 0.1 for the full record.