feat: [roadmap] 중장기 계획 메뉴 및 전용 페이지 개발

- 모델: AdminRoadmapPlan, AdminRoadmapMilestone
- 서비스: RoadmapPlanService, RoadmapMilestoneService
- FormRequest: Store/Update Plan/Milestone 4개
- 컨트롤러: Blade(RoadmapController), API(Plan/Milestone) 3개
- 라우트: web.php, api.php에 roadmap 라우트 추가
- Blade 뷰: 대시보드, 목록, 생성, 수정, 상세, 파셜 테이블 6개
- HTMX 기반 필터링/페이지네이션, 마일스톤 인라인 추가/토글

app/Http/Controllers/Api/Admin/Roadmap/RoadmapMilestoneController.php

@@ -0,0 +1,68 @@
+<?php
+
+namespace App\Http\Controllers\Api\Admin\Roadmap;
+
+use App\Http\Controllers\Controller;
+use App\Http\Requests\Roadmap\StoreMilestoneRequest;
+use App\Http\Requests\Roadmap\UpdateMilestoneRequest;
+use App\Services\Roadmap\RoadmapMilestoneService;
+use Illuminate\Http\JsonResponse;
+
+class RoadmapMilestoneController extends Controller
+{
+    public function __construct(
+        private readonly RoadmapMilestoneService $milestoneService
+    ) {}
+
+    public function byPlan(int $planId): JsonResponse
+    {
+        $milestones = $this->milestoneService->getMilestonesByPlan($planId);
+
+        return response()->json([
+            'success' => true,
+            'data' => $milestones,
+        ]);
+    }
+
+    public function store(StoreMilestoneRequest $request): JsonResponse
+    {
+        $milestone = $this->milestoneService->createMilestone($request->validated());
+
+        return response()->json([
+            'success' => true,
+            'message' => '마일스톤이 추가되었습니다.',
+            'data' => $milestone,
+        ]);
+    }
+
+    public function update(UpdateMilestoneRequest $request, int $id): JsonResponse
+    {
+        $this->milestoneService->updateMilestone($id, $request->validated());
+
+        return response()->json([
+            'success' => true,
+            'message' => '마일스톤이 수정되었습니다.',
+        ]);
+    }
+
+    public function destroy(int $id): JsonResponse
+    {
+        $this->milestoneService->deleteMilestone($id);
+
+        return response()->json([
+            'success' => true,
+            'message' => '마일스톤이 삭제되었습니다.',
+        ]);
+    }
+
+    public function toggle(int $id): JsonResponse
+    {
+        $milestone = $this->milestoneService->toggleStatus($id);
+
+        return response()->json([
+            'success' => true,
+            'message' => $milestone->status === 'completed' ? '마일스톤이 완료 처리되었습니다.' : '마일스톤이 미완료로 변경되었습니다.',
+            'data' => $milestone,
+        ]);
+    }
+}