feat(auth): Google OAuth, sealed session and default-deny gating (T9)

Gate the app behind Google sign-in restricted to verified @theodo.com
identities, and record the real reporter on filed defects (F9).

- nuxt-auth-utils for sealed cookie sessions + the Google OAuth handler,
  mounted at /auth/google/callback to match the registered redirect URI.
- isAllowedGoogleUser re-derives the domain from Google's verified email;
  the spoofable `hd` claim is deliberately ignored (DESIGN T9).
- Default-deny: server middleware 401s unauthenticated /api calls (health
  and the session endpoint excepted); a global route middleware redirects
  unauthenticated page navigations to /login.
- getReporter() now reads the session email instead of the dev stub.
- Env contract moves to nuxt-auth-utils names (NUXT_SESSION_PASSWORD,
  NUXT_OAUTH_GOOGLE_*); .env.example, compose and README updated.

Unit-tested: domain check (incl. hd-spoof + look-alike) and public-path
matching. Live OAuth round-trip pending manual verification.

app/middleware/auth.global.ts

@@ -0,0 +1,13 @@
+// Redirect unauthenticated visitors to the login page, and bounce already
+// logged-in users away from it (F9). Runs on both SSR and client navigations;
+// the server data API is gated independently in server/middleware/auth.ts.
+export default defineNuxtRouteMiddleware((to) => {
+  const { loggedIn } = useUserSession()
+
+  if (!loggedIn.value && to.path !== '/login') {
+    return navigateTo('/login')
+  }
+  if (loggedIn.value && to.path === '/login') {
+    return navigateTo('/')
+  }
+})