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.go

@@ -0,0 +1,71 @@
+// Package store is a stand-in for the real Postgres-backed tenant store.
+// The skeleton ships an in-memory implementation pre-seeded with one tenant
+// (acme) so portal middleware has something to resolve in local dev.
+// Replace with a pgx-backed implementation in the M4.1 follow-up PR.
+package store
+
+import (
+	"context"
+	"errors"
+	"sync"
+	"time"
+)
+
+var ErrNotFound = errors.New("tenant not found")
+
+type Tenant struct {
+	ID        string    `json:"id"`
+	Slug      string    `json:"slug"`
+	Name      string    `json:"name"`
+	Status    string    `json:"status"` // active | trial | frozen | archived | demo
+	Plan      string    `json:"plan"`   // starter | professional | enterprise
+	Products  []string  `json:"products"`
+	CreatedAt time.Time `json:"created_at"`
+}
+
+type Memory struct {
+	mu     sync.RWMutex
+	bySlug map[string]*Tenant
+	byID   map[string]*Tenant
+}
+
+func NewMemory() *Memory {
+	m := &Memory{
+		bySlug: make(map[string]*Tenant),
+		byID:   make(map[string]*Tenant),
+	}
+	seed := &Tenant{
+		ID:        "00000000-0000-0000-0000-000000000001",
+		Slug:      "acme",
+		Name:      "Acme Inc.",
+		Status:    "active",
+		Plan:      "professional",
+		Products:  []string{"certifai", "compliance"},
+		CreatedAt: time.Now().UTC(),
+	}
+	m.bySlug[seed.Slug] = seed
+	m.byID[seed.ID] = seed
+	return m
+}
+
+func (m *Memory) BySlug(_ context.Context, slug string) (*Tenant, error) {
+	m.mu.RLock()
+	defer m.mu.RUnlock()
+	t, ok := m.bySlug[slug]
+	if !ok {
+		return nil, ErrNotFound
+	}
+	cp := *t
+	return &cp, nil
+}
+
+func (m *Memory) ByID(_ context.Context, id string) (*Tenant, error) {
+	m.mu.RLock()
+	defer m.mu.RUnlock()
+	t, ok := m.byID[id]
+	if !ok {
+		return nil, ErrNotFound
+	}
+	cp := *t
+	return &cp, nil
+}