feat: implement post existence check and detail page

apps/frontend/src/app/add/page.tsx

@@ -1,6 +1,7 @@
 "use client";
 
 import { FormEvent, useEffect, useMemo, useState } from "react";
+import Link from "next/link";
 import Header from "../../components/header";
 
 type SourceType = "twitter" | "pixiv";
@@ -26,6 +27,22 @@ type Me = {
     role: "admin" | "writer" | "reader";
 };
 
+type UploadApiResponse = {
+    success: boolean;
+    message: string;
+    savedCount?: number;
+    failedCount?: number;
+};
+
+type ExistsApiResponse = {
+    success: boolean;
+    message: string;
+    exists: boolean;
+    source: SourceType | null;
+    postId: string | null;
+    documentId: string | null;
+};
+
 function detectSource(url: string): SourceType | null {
     if (/^https?:\/\/(www\.)?(x\.com|twitter\.com|fxtwitter\.com|fixupx\.com|vxwitter\.com)\//.test(url)) {
         return "twitter";
@@ -59,6 +76,7 @@ export default function AddPage() {
     const [lastFetchedUrl, setLastFetchedUrl] = useState("");
     const [viewerRole, setViewerRole] = useState<Me["role"] | "guest">("guest");
     const [loadingRole, setLoadingRole] = useState(true);
+    const [existingDetailId, setExistingDetailId] = useState<string | null>(null);
 
     const selectedCount = useMemo(
         () => selected.filter(Boolean).length,
@@ -110,6 +128,7 @@ export default function AddPage() {
         setLastFetchedUrl("");
         setError(null);
         setSuccess(null);
+        setExistingDetailId(null);
     }
 
     async function fetchPreview(targetUrl?: string) {
@@ -125,8 +144,25 @@ export default function AddPage() {
 
         setLoadingPreview(true);
         setSourceType(source);
+        setExistingDetailId(null);
 
         try {
+            const existsResponse = await fetch(`/api/post/exists?url=${encodeURIComponent(trimmedUrl)}`, {
+                cache: "no-store",
+            });
+
+            if (existsResponse.ok) {
+                const existsData = (await existsResponse.json()) as ExistsApiResponse;
+                if (existsData.exists) {
+                    setPreviewItems([]);
+                    setSelected([]);
+                    setLastFetchedUrl(trimmedUrl);
+                    setExistingDetailId(existsData.documentId);
+                    setSuccess(existsData.message);
+                    return;
+                }
+            }
+
             if (source === "twitter") {
                 const response = await fetch(`/api/tweet/fetch?url=${encodeURIComponent(trimmedUrl)}`, {
                     cache: "no-store",
@@ -255,17 +291,28 @@ export default function AddPage() {
                 }),
             });
 
-            if (!response.ok) {
-                const responseText = await response.text();
-                throw new Error(responseText || `업로드 실패: ${response.status}`);
+            let data: UploadApiResponse | null = null;
+            try {
+                data = (await response.json()) as UploadApiResponse;
+            } catch {
+                data = null;
             }
 
-            const uploadedCount = selectedCount;
-            setUrl("");
-            setAuthor("");
-            setTagsText("");
-            resetPreview();
-            setSuccess(`${uploadedCount}개 이미지 업로드를 요청했습니다.`);
+            const message = data?.message || `업로드 실패: ${response.status}`;
+
+            if (!response.ok || !data?.success) {
+                throw new Error(message);
+            }
+
+            const uploadedCount = data.savedCount ?? selectedCount;
+            if (uploadedCount > 0) {
+                setUrl("");
+                setAuthor("");
+                setTagsText("");
+                resetPreview();
+            }
+
+            setSuccess(message);
         } catch (submitError) {
             setError(submitError instanceof Error ? submitError.message : "업로드에 실패했습니다.");
         } finally {
@@ -351,7 +398,14 @@ export default function AddPage() {
                         </div>
 
                         {error ? <p className="text-sm text-red-600">{error}</p> : null}
-                        {success ? <p className="text-sm text-emerald-600">{success}</p> : null}
+                        {success ? <p className="text-sm text-emerald-600">{success} {existingDetailId ? (
+                            <Link
+                                href={`/detail/${existingDetailId}`}
+                                className="underline"
+                            >
+                                상세 페이지로 이동
+                            </Link>
+                        ) : null}</p> : null}
 
                         {loadingPreview ? (
                             <div className="border-l-2 border-border/80 pl-3 text-xs text-foreground/65">