feat: AI 辅助修改行程（自然语言调整 + 单活动替代推荐）

- 新增 refinePlan / suggestAlternativeItems 到 ai.ts
- 新增 POST /api/blindbox/plan/refine（整体行程调整）
- 新增 POST /api/blindbox/plan/suggest-item（单活动 AI 替代 + POI 搜索）
- BlindboxPlan 底部新增自然语言输入框（方案 A）
- 编辑 modal 内新增 AI 推荐替代方案卡片（方案 B）
- export searchPois 供 suggest-item 路由复用

src/app/api/blindbox/plan/refine/route.ts

@@ -0,0 +1,14 @@
+import { NextResponse } from "next/server";
+import { apiHandler, ApiError, requireUserId } from "@/lib/api";
+import { refinePlan } from "@/lib/ai";
+
+export const POST = apiHandler(async (req) => {
+  const { userId, instruction, days } = await req.json();
+  requireUserId(userId);
+  if (!instruction?.trim()) throw new ApiError("指令不能为空", 400);
+  if (!Array.isArray(days) || days.length === 0) throw new ApiError("days 无效", 400);
+
+  const newDays = await refinePlan(days, instruction);
+  if (!newDays) throw new ApiError("AI 调整失败，请重试", 500);
+  return NextResponse.json({ days: newDays });
+});

src/app/api/blindbox/plan/suggest-item/route.ts

@@ -0,0 +1,49 @@
+import { NextResponse } from "next/server";
+import { apiHandler, ApiError } from "@/lib/api";
+import { suggestAlternativeItems } from "@/lib/ai";
+import { searchPois } from "@/lib/blindboxPlanGen";
+
+export const POST = apiHandler(async (req) => {
+  const { activity, time, location } = await req.json();
+  if (!activity) throw new ApiError("activity 不能为空", 400);
+
+  const alts = await suggestAlternativeItems(activity, time ?? "");
+  if (!alts) throw new ApiError("AI 推荐失败，请重试", 500);
+
+  // Parse location "lng,lat"
+  let anchorLat = 31.23, anchorLng = 121.47; // default Shanghai
+  if (location) {
+    const [lng, lat] = location.split(",").map(Number);
+    if (!isNaN(lat) && !isNaN(lng)) { anchorLat = lat; anchorLng = lng; }
+  }
+
+  // Parallel POI search with per-item fallback
+  const results = await Promise.all(
+    alts.map(async (alt) => {
+      try {
+        const pois = await searchPois(alt.searchQuery, "place", anchorLat, anchorLng);
+        const top = pois[0];
+        if (top) {
+          return {
+            activity: alt.activity,
+            poi: top.name,
+            address: top.address,
+            lat: top.lat,
+            lng: top.lng,
+            reason: alt.reason,
+          };
+        }
+      } catch { /* ignore, use fallback */ }
+      return {
+        activity: alt.activity,
+        poi: alt.searchQuery,
+        address: "",
+        lat: 0,
+        lng: 0,
+        reason: alt.reason,
+      };
+    }),
+  );
+
+  return NextResponse.json({ suggestions: results });
+});