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/server/tenants_test.go

@@ -3,6 +3,7 @@ package server_test
 import (
 	"net/http"
 	"testing"
+	"time"
 
 	"gitea.meghsakha.com/platform/tenant-registry/internal/store"
 )
@@ -119,3 +120,42 @@ func TestCancelTenant(t *testing.T) {
 		}
 	})
 }
+
+func TestCreateTenant_setsTrialEndsAt(t *testing.T) {
+	eachStore(t, func(t *testing.T, h *testHarness) {
+		_, body := h.do("POST", "/v1/tenants", map[string]any{
+			"slug": "trial-ends-co", "name": "Trial Ends Co.",
+		})
+		out := decode[struct {
+			Tenant *store.Tenant `json:"tenant"`
+		}](t, body)
+		if out.Tenant.Status != "trial" {
+			t.Fatalf("status = %q, want trial", out.Tenant.Status)
+		}
+		if out.Tenant.TrialEndsAt == nil {
+			t.Fatal("trial_ends_at is nil; should be ~14 days from now")
+		}
+		// Sanity-check: ends_at is in the future, within 13.5-14.5 days.
+		delta := time.Until(*out.Tenant.TrialEndsAt)
+		if delta < 13*24*time.Hour || delta > 15*24*time.Hour {
+			t.Errorf("trial_ends_at offset = %v, want ~14d", delta)
+		}
+	})
+}
+
+func TestCreateTenant_demoKindHasNoTrialEnd(t *testing.T) {
+	eachStore(t, func(t *testing.T, h *testHarness) {
+		_, body := h.do("POST", "/v1/tenants", map[string]any{
+			"slug": "demo-co", "name": "Demo", "kind": "demo",
+		})
+		out := decode[struct {
+			Tenant *store.Tenant `json:"tenant"`
+		}](t, body)
+		if out.Tenant.Status != "demo" {
+			t.Errorf("status = %q, want demo", out.Tenant.Status)
+		}
+		if out.Tenant.TrialEndsAt != nil {
+			t.Errorf("trial_ends_at = %v, want nil for demo kind", out.Tenant.TrialEndsAt)
+		}
+	})
+}