fix(i18n): add next-intl middleware for locale routing

The middleware was deleted in the Next.js 16 upgrade but is still required
by next-intl to handle locale detection and URL rewriting. Without it,
/auth/sign-in can't resolve to [locale=de]/auth/sign-in → 404.

Uses createMiddleware from next-intl/middleware with the shared routing config.

Dockerfile

@@ -4,7 +4,7 @@ WORKDIR /app
 
 # --- Install + Build in one stage ---
 FROM base AS builder
-ARG CACHE_BUST=3
+ARG CACHE_BUST=4
 COPY . .
 RUN pnpm install --no-frozen-lockfile
 ENV NEXT_TELEMETRY_DISABLED=1