feat(tenancies-p5): sidebar entry + 404 top-level page + API module gate

- Dashboard layout resolves tenanciesModuleByPort server-side (one
  isTenanciesModuleEnabled call per port the user has access to) and
  passes the map through AppShell → Sidebar. Atomic SSR — no
  flicker of the nav entry in/out after hydration.
- Sidebar gains NavItemGated.requiresTenanciesModule. The Tenancies
  entry (KeyRound icon, immediately below Berths) only renders when
  the currently-active port has the flag flipped on. Per-port live
  switch fires when the rep toggles ports without reload.
- /[portSlug]/tenancies + /[portSlug]/tenancies/[id] both call
  isTenanciesModuleEnabled and notFound() when disabled — guards
  against direct URL access even when the sidebar is hidden.
- API routes (/api/v1/tenancies, /[id], /berths/[id]/tenancies)
  prepended with assertTenanciesModuleEnabled — matches design §
  "All routes ... return 404 when off". NotFoundError maps to 404.
- Existing tenancy API tests get a makePortWithTenancies() helper
  (calls enableTenanciesModule after makePort) so the gate is
  satisfied. Affects 2 test files (16 tests retargeted).

Verified: tsc clean, 1493/1493 vitest.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/app/(dashboard)/layout.tsx

@@ -17,6 +17,7 @@ import { RealtimeToasts } from '@/components/shared/realtime-toasts';
 import { WebVitalsReporter } from '@/components/shared/web-vitals-reporter';
 import { classifyFormFactor } from '@/lib/form-factor';
 import { getPortBrandingConfig } from '@/lib/services/port-config';
+import { isTenanciesModuleEnabled } from '@/lib/services/tenancies-module.service';
 
 export default async function DashboardLayout({ children }: { children: React.ReactNode }) {
   const headerList = await headers();
@@ -73,6 +74,21 @@ export default async function DashboardLayout({ children }: { children: React.Re
   );
   const portLogoUrls: Record<string, string | null> = Object.fromEntries(portBrandingEntries);
 
+  // Per-port tenancies-module gate. Hidden by default; flips on either by
+  // the admin switch (Operations) OR the lazy auto-enable on first row.
+  // Resolved server-side so the sidebar nav SSRs in/out atomically with
+  // the layout instead of flickering after a client-side fetch.
+  const tenanciesModuleEntries = await Promise.all(
+    ports.map(async (p) => {
+      try {
+        return [p.id, await isTenanciesModuleEnabled(p.id)] as const;
+      } catch {
+        return [p.id, false] as const;
+      }
+    }),
+  );
+  const tenanciesModuleByPort: Record<string, boolean> = Object.fromEntries(tenanciesModuleEntries);
+
   return (
     <QueryProvider>
       <PortProvider ports={ports} defaultPortId={ports[0]?.id ?? null}>
@@ -95,6 +111,7 @@ export default async function DashboardLayout({ children }: { children: React.Re
               user={user}
               ports={ports}
               portLogoUrls={portLogoUrls}
+              tenanciesModuleByPort={tenanciesModuleByPort}
               initialFormFactor={initialFormFactor}
             >
               {children}