支付系统核心：配额限制与用户付费状态管理

- 添加 tokio 依赖用于异步测试
- 启用 User 结构体，添加 is_paid/is_admin/paid_expires_at 字段
- 添加 UpdatePaymentRequest 请求体
- insert_weather_data 集成配额检查逻辑
- 新增 get_user_by_id、count_user_weather_data、update_user_payment_status
- 添加 payment_fields 数据库迁移脚本

Cargo.lock

@@ -2079,6 +2079,7 @@ dependencies = [
  "serde",
  "serde_json",
  "sqlx",
+ "tokio",
 ]
 
 [[package]]
@@ -2718,9 +2719,21 @@ dependencies = [
  "signal-hook-registry",
  "slab",
  "socket2 0.6.0",
+ "tokio-macros",
  "windows-sys 0.59.0",
 ]
 
+[[package]]
+name = "tokio-macros"
+version = "2.5.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "6e06d43f1345a3bcd39f6a56dbb7dcab2ba47e68e8ac134855e7e2bdbaf8cab8"
+dependencies = [
+ "proc-macro2",
+ "quote",
+ "syn",
+]
+
 [[package]]
 name = "tokio-native-tls"
 version = "0.3.1"