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

apps/web/middleware.ts

@@ -0,0 +1,15 @@
+import createMiddleware from 'next-intl/middleware';
+
+import { routing } from '@kit/i18n/routing';
+
+export default createMiddleware(routing);
+
+export const config = {
+  matcher: [
+    // Match all pathnames except:
+    // - API routes (/api/...)
+    // - Next.js internals (/_next/...)
+    // - Static files with extensions
+    '/((?!api|_next|.*\\..*).*)',
+  ],
+};