feat(store): set trial_ends_at on tenant create

CreateTenant now defaults trial_ends_at to NOW() + 14 days when the
new tenant lands in status='trial'. Demo-kind tenants get
status='demo' (per PLATFORM_ARCHITECTURE.md §5d) and trial_ends_at
stays NULL — those flow through the M13.2 demo-provisioning path.

Both store implementations (Memory + Postgres) updated; tests assert
the 14-day window for customers and the absent end for demo kind.

Unblocks M12.1 (portal trial banner can render a real countdown).

Refs: M4.1 + M12.1

internal/store/memory.go

@@ -69,16 +69,26 @@ func (m *Memory) CreateTenant(_ context.Context, in TenantCreate) (*Tenant, erro
 		return nil, ErrConflict
 	}
 	now := time.Now().UTC()
+	kind := firstNonEmpty(in.Kind, "customer")
+	status := "trial"
+	var trialEnds *time.Time
+	if kind == "demo" {
+		status = "demo"
+	} else {
+		end := now.Add(14 * 24 * time.Hour)
+		trialEnds = &end
+	}
 	t := &Tenant{
-		ID:         uuid.NewString(),
-		Slug:       in.Slug,
-		Name:       in.Name,
-		Status:     "trial",
-		Kind:       firstNonEmpty(in.Kind, "customer"),
-		Plan:       firstNonEmpty(in.Plan, "starter"),
-		SalesOwner: in.SalesOwner,
-		CreatedAt:  now,
-		UpdatedAt:  now,
+		ID:          uuid.NewString(),
+		Slug:        in.Slug,
+		Name:        in.Name,
+		Status:      status,
+		Kind:        kind,
+		Plan:        firstNonEmpty(in.Plan, "starter"),
+		SalesOwner:  in.SalesOwner,
+		TrialEndsAt: trialEnds,
+		CreatedAt:   now,
+		UpdatedAt:   now,
 	}
 	m.tenants[t.ID] = t
 	m.bySlug[t.Slug] = t.ID