CHORE(merge): merge from develop

- Initial setup and all features from develop branch
- Includes: auth, deploy, docker, style fixes
- K3S deployment configuration

nextjs/app/api/files/download-url/route.ts

@@ -0,0 +1,32 @@
+import { NextRequest, NextResponse } from 'next/server';
+import { generateSignedUrl } from '@/lib/s3';
+
+export async function POST(request: NextRequest) {
+  try {
+    const body = await request.json();
+    const { fileKey, fileName } = body;
+
+    if (!fileKey) {
+      return NextResponse.json(
+        { success: false, message: 'fileKey가 필요합니다.' },
+        { status: 400 }
+      );
+    }
+
+    const downloadUrl = await generateSignedUrl(fileKey, { fileName });
+
+    return NextResponse.json({
+      success: true,
+      data: {
+        downloadUrl,
+      },
+    });
+  } catch (err) {
+    const errorMessage = err instanceof Error ? err.message : '다운로드 URL 생성에 실패했습니다.';
+    return NextResponse.json(
+      { success: false, message: errorMessage },
+      { status: 500 }
+    );
+  }
+}
+