feat: 升级日志系统为 tracing 结构化日志

- 替换 env_logger 为 tracing + tracing-subscriber
- 统一使用 tracing::{debug, error, info, warn} 日志宏
- 消除所有 eprintln!/println 调用
- 添加 JSON 格式日志输出便于 ELK Stack 收集
- 添加文件日志轮转（每天一个新文件 ./logs/rust-backend-{date}.log）
- 日志等级过滤：rust_backend=info, actix_web=info, sqlx=warn
- 更新 IMPROVEMENTS.md 标记 6.1 日志系统两项完成

Cargo.toml

@@ -8,7 +8,9 @@ actix-files = "0.6.8"
 actix-web = {version = "4.11.0", features=["openssl"]}
 chrono = {version = "0.4.41", features=["serde"]}
 dotenvy = "0.15.7"
-env_logger = "0.11.8"
+tracing = "0.1"
+tracing-subscriber = { version = "0.3", features = ["env-filter", "json"] }
+tracing-appender = "0.2"
 error = "0.1.9"
 include_dir = "0.7.4"
 jsonwebtoken = "9.3.1"