feat: county sync on monitor page + in-app notification system

- GET /api/eterra/counties — distinct county list from GisUat
- POST /api/eterra/sync-county — background sync all UATs in a county
  (TERENURI + CLADIRI + INTRAVILAN), magic mode for enriched UATs,
  concurrency guard, creates notification on completion
- In-app notification service (KeyValueStore, CRUD, unread count)
- GET/PATCH /api/notifications/app — list and mark-read endpoints
- NotificationBell component in header with popover + polling
- Monitor page: county select dropdown + SyncTestButton with customBody

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/shared/components/layout/header.tsx

@@ -13,6 +13,7 @@ import {
 import { useAuth } from "@/core/auth";
 import { signIn, signOut } from "next-auth/react";
 import { ThemeToggle } from "@/shared/components/common/theme-toggle";
+import { NotificationBell } from "./notification-bell";
 
 interface HeaderProps {
   onToggleSidebar?: () => void;
@@ -35,6 +36,7 @@ export function Header({ onToggleSidebar }: HeaderProps) {
       </div>
 
       <div className="flex items-center gap-3">
+        <NotificationBell />
         <ThemeToggle />
 
         <DropdownMenu>