feat(db): add usage tracking schema and types

Add database tables for proxy request logs and model pricing.
Extend Provider and error types to support usage statistics.

src-tauri/src/proxy/usage/mod.rs

@@ -0,0 +1,11 @@
+//! Proxy Usage Tracking Module
+//!
+//! 提供 API 请求的使用量跟踪、成本计算和日志记录功能
+
+pub mod calculator;
+pub mod logger;
+pub mod parser;
+
+pub use calculator::{CostBreakdown, CostCalculator, ModelPricing};
+pub use logger::{RequestLog, UsageLogger};
+pub use parser::{ApiType, TokenUsage};