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

cmd/server/main.go

@@ -12,6 +12,7 @@ import (
 
 	"gitea.meghsakha.com/platform/tenant-registry/internal/config"
 	"gitea.meghsakha.com/platform/tenant-registry/internal/server"
+	"gitea.meghsakha.com/platform/tenant-registry/internal/store"
 )
 
 func main() {
@@ -24,10 +25,27 @@ func main() {
 		os.Exit(1)
 	}
 
-	mux := server.NewRouter(cfg, logger)
+	bootCtx, cancelBoot := context.WithTimeout(context.Background(), 15*time.Second)
+	defer cancelBoot()
+
+	var s store.Store
+	if cfg.DatabaseURL == "" {
+		slog.Warn("DATABASE_URL not set — running with in-memory store (dev only)")
+		s = store.NewMemory()
+	} else {
+		pg, err := store.NewPostgres(bootCtx, cfg.DatabaseURL)
+		if err != nil {
+			slog.Error("postgres connect failed", "err", err)
+			os.Exit(1)
+		}
+		s = pg
+	}
+	defer s.Close()
+
+	handler := server.NewRouter(&server.Server{Cfg: cfg, Log: logger, Store: s})
 	srv := &http.Server{
 		Addr:         cfg.Addr,
-		Handler:      mux,
+		Handler:      handler,
 		ReadTimeout:  10 * time.Second,
 		WriteTimeout: 30 * time.Second,
 		IdleTimeout:  60 * time.Second,
@@ -37,7 +55,7 @@ func main() {
 	defer stop()
 
 	go func() {
-		slog.Info("tenant-registry listening", "addr", cfg.Addr, "env", cfg.Env)
+		slog.Info("tenant-registry listening", "addr", cfg.Addr, "env", cfg.Env, "store", storeKind(s))
 		if err := srv.ListenAndServe(); err != nil && !errors.Is(err, http.ErrServerClosed) {
 			slog.Error("server crashed", "err", err)
 			os.Exit(1)
@@ -54,3 +72,14 @@ func main() {
 	}
 	slog.Info("bye")
 }
+
+func storeKind(s store.Store) string {
+	switch s.(type) {
+	case *store.Memory:
+		return "memory"
+	case *store.Postgres:
+		return "postgres"
+	default:
+		return "unknown"
+	}
+}