feat: AI岗位专区 — 5个AI岗位置顶 + 首页分组展示

- schema: HotPosition 新增 category 字段 (ai/traditional)
- positions: 5 AI岗位 (AI算法/大模型应用/Prompt/AI产品/AI运维) + 7传统岗位
- frontend: 首页拆分 "🔥 AI热门岗位" 置顶高亮 + "更多岗位" 折叠
- ai服务: 新增 primaryFallbackModel (sensenova-6.7-flash-lite) 降级链路

.omo/run-continuation/ses_130b21833ffe0epQvGBjpGtogZ.json

@@ -0,0 +1,11 @@
+{
+  "sessionID": "ses_130b21833ffe0epQvGBjpGtogZ",
+  "updatedAt": "2026-06-17T05:54:06.381Z",
+  "sources": {
+    "background-task": {
+      "state": "active",
+      "reason": "2 background task(s) active",
+      "updatedAt": "2026-06-17T05:54:06.381Z"
+    }
+  }
+}