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.

server/utils/authPaths.ts

@@ -0,0 +1,12 @@
+// Which `/api` routes bypass the default-deny auth middleware (F9).
+//
+// `/api/health` must answer for the orchestrator's liveness probe before any
+// session exists; `/api/_auth/*` is nuxt-auth-utils' own session endpoint that
+// the client uses to read/clear the session. Everything else under `/api`
+// requires an authenticated session.
+const PUBLIC_API_PREFIXES = ['/api/health', '/api/_auth/']
+
+export function isPublicApiPath(path: string): boolean {
+  const pathname = path.split('?')[0]!
+  return PUBLIC_API_PREFIXES.some((prefix) => pathname.startsWith(prefix))
+}