feat(api): M4.2 — full REST surface + pgx-backed Postgres store

Replaces the M5.1-skeleton handler set with the M4.2 spec from
IMPLEMENTATION_PLAN.md:

Endpoints (authoritative shape in openapi.yaml):
  POST   /v1/tenants
  GET    /v1/tenants/{id}
  GET    /v1/tenants/by-slug/{slug}
  POST   /v1/tenants/{id}/activate
  POST   /v1/tenants/{id}/cancel
  GET    /v1/entitlements?tenant_id=...
  GET    /v1/catalog
  POST   /v1/catalog/request
  POST   /v1/catalog/trial-request
  POST   /v1/api-keys                       returns plaintext ONCE
  GET    /v1/api-keys?tenant_id=...
  DELETE /v1/api-keys/{id}
  POST   /v1/internal/api-keys/verify       always 200; valid: bool
  POST   /v1/audit
  GET    /v1/audit?{tenant_id,product,actor_id,action,since,until,limit,cursor}

Architecture:
  internal/store/store.go        Store interface (CRUD + audit + ping)
  internal/store/memory.go       in-process impl, used when DATABASE_URL
                                 is empty (seed acme tenant, no migrations)
  internal/store/postgres.go     pgxpool impl against the M4.1 schema
  internal/server/server.go      router + healthz/readyz
  internal/server/{tenants,catalog,apikeys,audit}.go
                                 per-concern handlers (≤250 LoC each)
  internal/server/helpers.go     writeJSON/writeError/error mapping/log mw
  openapi.yaml                   3.1 spec; openapi_test.go is the contract gate

API keys:
  Plaintext format 'bp_<22-char base64>'. Prefix bp_<8> stored for UI.
  Hash is argon2id(salt, time=1, mem=64MB, threads=4, len=32) encoded as
  'argon2id|<salt-b64>|<hash-b64>'. Format-tagged so we can rotate
  parameters without re-keying. Verify is constant-time.

Store selection:
  cmd/server picks Postgres when DATABASE_URL is set, otherwise Memory.
  Both implementations are exercised by the same eachStore test harness —
  parity is enforced.

Audit:
  Every state-changing endpoint emits via s.emitAudit() (fire-and-forget).
  audit_log uses ON DELETE SET NULL on tenant_id so forensic history
  outlives tenant deletes (per M4.1 schema).

Routing constraint:
  Go 1.22 ServeMux can't disambiguate /v1/tenants/{id}/products from
  /v1/tenants/by-slug/{slug=products}. Per-tenant subresources moved to
  query-param top-level paths: /v1/entitlements?tenant_id=… and
  /v1/api-keys?tenant_id=….

Tests:
  Every endpoint exercised against both Memory and Postgres via the
  eachStore harness. Includes happy paths, validation errors, conflicts,
  404s, auto-audit-emit assertion. testcontainers-go for the postgres
  harness; gated by -short.

  TestOpenAPISpec is the contract gate: every documented operation must
  resolve against the router. (kin-openapi v0.138.0.)

Refs: M4.2

.env.example

@@ -1,11 +1,11 @@
 # tenant-registry — local dev environment.
-#
-# Copy to .env.local (gitignored) and edit. The service reads env vars
-# directly via internal/config; this file is just documentation.
+# Copy to .env.local (gitignored).
 
 APP_ENV=dev
 ADDR=:8090
 KEYCLOAK_ISSUER=http://localhost:8080/realms/breakpilot-dev
 
-# Postgres DSN — unused by the skeleton (in-memory store). Wired up in M4.1.
+# Postgres DSN. Leave empty for in-memory store (the seeded acme tenant
+# only; data lost on restart). Set to use the dev-stack Postgres + run
+# `make migrate-up` first.
 # DATABASE_URL=postgres://platform:platform-dev-pass@localhost:5432/platform?sslmode=disable