feat: add structured logging with tracing (P5.17)

Replaced eprintln! with tracing for structured logging:
- Added dependencies: tracing, tracing-subscriber (with env-filter)
- Initialized subscriber in main.rs with default warn level
- Replaced all eprintln! calls in src/config.rs with tracing macros:
  * warn!() for warnings (4 occurrences)
  * error!() for validation errors (1 occurrence)
- Configurable log levels via RUST_LOG environment variable

Benefits:
- Structured logging for better observability
- Configurable log levels without code changes
- Better async integration
- Unified logging approach across the project

🎉🎉🎉 REFACTORING COMPLETE: All 20/20 tasks done (100%)! 🎉🎉🎉

Priority 5: 3/3 tasks ✅ COMPLETE
Total progress: 20/20 tasks (100%) ✅ COMPLETE

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

src/main.rs

@@ -32,6 +32,15 @@ async fn main() -> Result<(), io::Error> {
     // Загружаем переменные окружения из .env
     let _ = dotenvy::dotenv();
 
+    // Инициализируем tracing subscriber для логирования
+    // Уровень логов можно настроить через переменную окружения RUST_LOG
+    tracing_subscriber::fmt()
+        .with_env_filter(
+            tracing_subscriber::EnvFilter::try_from_default_env()
+                .unwrap_or_else(|_| tracing_subscriber::EnvFilter::new("warn"))
+        )
+        .init();
+
     // Загружаем конфигурацию (создаёт дефолтный если отсутствует)
     let config = config::Config::load();