feat(server): tenant-registry skeleton boots against dev stack

Minimal Go service: /healthz + /v1/tenants/by-slug/:slug + /v1/tenants/:id with an in-memory store seeded with the acme tenant. Stdlib-only; pgx + JWT validation land in M4.1 follow-up.

internal/store/memory_test.go

@@ -0,0 +1,64 @@
+package store
+
+import (
+	"context"
+	"errors"
+	"testing"
+)
+
+func TestMemory_seededAcme(t *testing.T) {
+	m := NewMemory()
+	ctx := context.Background()
+
+	t.Run("by slug returns seed", func(t *testing.T) {
+		got, err := m.BySlug(ctx, "acme")
+		if err != nil {
+			t.Fatal(err)
+		}
+		if got.Slug != "acme" {
+			t.Errorf("slug = %q, want acme", got.Slug)
+		}
+		if got.Status != "active" {
+			t.Errorf("status = %q, want active", got.Status)
+		}
+		if len(got.Products) != 2 {
+			t.Errorf("products = %v, want [certifai compliance]", got.Products)
+		}
+	})
+
+	t.Run("by id returns seed", func(t *testing.T) {
+		got, err := m.ByID(ctx, "00000000-0000-0000-0000-000000000001")
+		if err != nil {
+			t.Fatal(err)
+		}
+		if got.Slug != "acme" {
+			t.Errorf("slug = %q, want acme", got.Slug)
+		}
+	})
+
+	t.Run("missing slug returns ErrNotFound", func(t *testing.T) {
+		_, err := m.BySlug(ctx, "nope")
+		if !errors.Is(err, ErrNotFound) {
+			t.Errorf("err = %v, want ErrNotFound", err)
+		}
+	})
+
+	t.Run("missing id returns ErrNotFound", func(t *testing.T) {
+		_, err := m.ByID(ctx, "deadbeef")
+		if !errors.Is(err, ErrNotFound) {
+			t.Errorf("err = %v, want ErrNotFound", err)
+		}
+	})
+
+	t.Run("returned tenant is a copy, not the stored pointer", func(t *testing.T) {
+		got, err := m.BySlug(ctx, "acme")
+		if err != nil {
+			t.Fatal(err)
+		}
+		got.Name = "mutated"
+		got2, _ := m.BySlug(ctx, "acme")
+		if got2.Name == "mutated" {
+			t.Error("store leaked internal pointer; caller could mutate seeded state")
+		}
+	})
+}