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/lib/ai.ts

@@ -230,6 +230,107 @@ ${Object.entries(ctx.candidates)
   }
 }
 
+// ---------------------------------------------------------------------------
+// refinePlan: adjust an existing plan based on a natural-language instruction
+// ---------------------------------------------------------------------------
+
+const REFINE_PLAN_SYSTEM_PROMPT = `你是一个周末行程调整助手。根据用户指令修改现有行程。规则：
+1. 只改用户明确要求的部分，其余原样保留
+2. 时间安排合理，活动间留交通时间
+3. 若需新增活动，poi/address 可留空字符串，lat/lng 填 0
+4. 严格按输入的 JSON 结构输出完整 days 数组`;
+
+export async function refinePlan(
+  days: import("@/types").WeekendPlanData[],
+  instruction: string,
+): Promise<import("@/types").WeekendPlanData[] | null> {
+  try {
+    const client = getClient();
+    const response = await client.chat.completions.create({
+      model: "deepseek-chat",
+      messages: [
+        { role: "system", content: REFINE_PLAN_SYSTEM_PROMPT },
+        {
+          role: "user",
+          content: `当前行程：\n${JSON.stringify(days)}\n\n用户指令：${instruction}`,
+        },
+      ],
+      response_format: { type: "json_object" },
+      temperature: 0.5,
+      max_tokens: 2000,
+    });
+
+    const text = response.choices[0]?.message?.content;
+    if (!text) return null;
+
+    const parsed = JSON.parse(text);
+    const result = parsed.days ?? parsed;
+    if (!Array.isArray(result) || result.length === 0) return null;
+    if (!result.every((d: unknown) => {
+      if (typeof d !== "object" || d === null) return false;
+      const day = d as Record<string, unknown>;
+      return typeof day.date === "string" && Array.isArray(day.items);
+    })) return null;
+
+    return result as import("@/types").WeekendPlanData[];
+  } catch {
+    return null;
+  }
+}
+
+// ---------------------------------------------------------------------------
+// suggestAlternativeItems: recommend 3 alternatives for a single activity
+// ---------------------------------------------------------------------------
+
+const SUGGEST_ALT_SYSTEM_PROMPT = `你是一个周末活动推荐助手。根据当前活动推荐 3 个类似但不同的替代方案，用于替换行程中的该活动。
+
+每个替代方案需要包含：
+- activity: 活动描述（5-15字，口语化）
+- searchQuery: 高德地图搜索关键词
+- reason: 推荐理由（一句话）
+
+只返回 JSON：{ "alternatives": [{ "activity", "searchQuery", "reason" }] }`;
+
+export async function suggestAlternativeItems(
+  activity: string,
+  time: string,
+): Promise<Array<{ activity: string; searchQuery: string; reason: string }> | null> {
+  try {
+    const client = getClient();
+    const response = await client.chat.completions.create({
+      model: "deepseek-chat",
+      messages: [
+        { role: "system", content: SUGGEST_ALT_SYSTEM_PROMPT },
+        {
+          role: "user",
+          content: `当前活动：${activity}${time ? `（时间：${time}）` : ""}`,
+        },
+      ],
+      response_format: { type: "json_object" },
+      temperature: 0.8,
+      max_tokens: 400,
+    });
+
+    const text = response.choices[0]?.message?.content;
+    if (!text) return null;
+
+    const parsed = JSON.parse(text);
+    if (!Array.isArray(parsed.alternatives)) return null;
+
+    return parsed.alternatives
+      .filter(
+        (a: unknown) =>
+          typeof a === "object" &&
+          a !== null &&
+          typeof (a as Record<string, unknown>).activity === "string" &&
+          typeof (a as Record<string, unknown>).searchQuery === "string",
+      )
+      .slice(0, 3) as Array<{ activity: string; searchQuery: string; reason: string }>;
+  } catch {
+    return null;
+  }
+}
+
 // ---------------------------------------------------------------------------
 // Generic tool-calling agent loop
 // ---------------------------------------------------------------------------