feat(tenants): provision the product tenant with the registry UUID on create

Closes the tenant-anchor divergence that blocked the Auth-5 sdk/backend flips.

The registry is the authority for tenant identity (model B2), but nothing ever
told the product about a new tenant. The anchors drifted: the registry held
acme/matrix-acme while the product database held only the legacy seed tenant
9282a473. With the SDK gate enabled its TenantResolver does GetTenantBySlug and
would 403 EVERY authenticated request, because no real user's org slug existed
locally.

New internal/product port, mirroring keycloak.Adapter: handlers depend on the
interface, main wires HTTPProvisioner when PRODUCT_API_URL is set and
NoopProvisioner otherwise, so an unconfigured deployment still creates tenants.
Tenant creation now also provisions the product tenant with OUR uuid, using the
same best-effort contract as Keycloak provisioning: a failure does not roll the
tenant back, it emits a product.provision_failed audit event so the divergence
is traceable. Success emits product.tenant_provisioned.

A 409 from the product counts as success — onboarding may retry and the
product's insert is idempotent on the primary key (compliance#217).

Server.productProvisioner() guarantees the documented never-nil invariant;
tests construct Server directly and would otherwise panic mid-tenant-creation.

5 tests incl. the point of the whole port (the registry UUID is what gets sent).
Full suite green with -race, coverage 71.4% (gate 70).

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01CNdLL9BdsWm7MCyui5ffPD

internal/server/server.go

@@ -12,6 +12,7 @@ import (
 	"gitea.meghsakha.com/platform/tenant-registry/internal/authn"
 	"gitea.meghsakha.com/platform/tenant-registry/internal/config"
 	"gitea.meghsakha.com/platform/tenant-registry/internal/keycloak"
+	"gitea.meghsakha.com/platform/tenant-registry/internal/product"
 	"gitea.meghsakha.com/platform/tenant-registry/internal/store"
 )
 
@@ -20,8 +21,19 @@ type Server struct {
 	Cfg      *config.Config
 	Log      *slog.Logger
 	Store    store.Store
-	Keycloak keycloak.Adapter // never nil — main wires Mock when KC env is unset
-	Auth     *authn.Verifier  // nil ⇒ AUTH_ENABLED=false, API is open (dev only)
+	Keycloak keycloak.Adapter    // never nil — main wires Mock when KC env is unset
+	Auth     *authn.Verifier     // nil ⇒ AUTH_ENABLED=false, API is open (dev only)
+	Product  product.Provisioner // never nil — main wires Noop when PRODUCT_API_URL is unset
+}
+
+// productProvisioner guarantees the "never nil" invariant the struct documents.
+// Tests construct Server directly and would otherwise panic; an unset provisioner
+// simply means no downstream provisioning, never a crash mid-tenant-creation.
+func (s *Server) productProvisioner() product.Provisioner {
+	if s.Product == nil {
+		return product.NoopProvisioner{}
+	}
+	return s.Product
 }
 
 // NewRouter builds the http.Handler with logging middleware applied.