feat(01-03): add Edge middleware + internal validate-token API route

- src/middleware.ts: Edge-compatible token validation via fetch() (no Drizzle import)
- src/app/api/internal/validate-token/route.ts: Node.js route queries clients.token via Drizzle
- Invalid tokens rewrite to /not-found (404); matcher scoped to /c/:path*

src/app/api/internal/validate-token/route.ts

@@ -0,0 +1,28 @@
+import { NextRequest, NextResponse } from 'next/server';
+import { eq } from 'drizzle-orm';
+import { db } from '@/db';
+import { clients } from '@/db/schema';
+
+export async function GET(request: NextRequest) {
+  const token = request.nextUrl.searchParams.get('token');
+
+  if (!token) {
+    return NextResponse.json({ valid: false }, { status: 400 });
+  }
+
+  try {
+    const rows = await db
+      .select({ id: clients.id })
+      .from(clients)
+      .where(eq(clients.token, token))
+      .limit(1);
+
+    if (rows.length === 0) {
+      return NextResponse.json({ valid: false }, { status: 404 });
+    }
+
+    return NextResponse.json({ valid: true }, { status: 200 });
+  } catch {
+    return NextResponse.json({ valid: false }, { status: 500 });
+  }
+}