fix: useful_count type + revert resp.Result (use report_json)

1. Feedback端点  类型修复 (core.go)
   - 之前传 map[string]int，Update() 只处理 map[string]string/map[string]interface{}
   - int 被当作 string case 处理， 被丢弃
   - 修复: map[string]interface{}{"": 1}

2. IPC 搜索字段名 (lancedb_ipc.go)
   - Rust send_ok 用 report_json，不是 result
   - 错误改用 resp.Result 导致 IPC 结果永远为空
   - 回退到 resp.ReportJSON (这是正确的)
   - 加 Result 字段只是为了日志可见性（不被 Rust 填充）

验证: recall_count=6, useful_count=1 确认写入 LanceDB

go/internal/api/routes/core.go

@@ -268,7 +268,7 @@ func (a *API) Feedback(w http.ResponseWriter, r *http.Request) {
 		return
 	}
 	field := "useful_count"
-	val := map[string]int{"$inc": 1}
+	val := map[string]interface{}{"$inc": 1}
 	if !req.Useful {
 		field = "not_useful_count"
 	}

go/internal/storage/lancedb_ipc.go

@@ -157,12 +157,12 @@ func (rc *RustLanceDBClient) Search(table string, vector []float32, topK int, na
 		Tier           string  `json:"tier"`
 		LastRecalledAt string  `json:"last_recalled_at"`
 	}
-	json.Unmarshal([]byte(resp.Result), &raw)
+	json.Unmarshal([]byte(resp.ReportJSON), &raw)
 	if len(raw) == 0 {
-		log.Printf("[ipc] Search: resp.Result unmarshal gave 0 results (ReportJSON=%q), falling back to localSearch", resp.ReportJSON)
+		log.Printf("[ipc] Search: resp.ReportJSON unmarshal gave 0 results, falling back to localSearch (Result=%q)", resp.Result)
 		return rc.localSearch(vector, topK, namespaceFilter), nil
 	}
-	log.Printf("[ipc] Search: got %d results from Rust IPC (ReportJSON=%q)", len(raw), resp.ReportJSON)
+	log.Printf("[ipc] Search: got %d results from Rust IPC (Result=%q)", len(raw), resp.Result)
 
 	// 解析时间（Rust 返回 RFC3339 字符串 → time.Time）
 	parseTime := func(s string) time.Time {