fix: suppress all compiler warnings

- mcp_server.rs: #[allow(dead_code)] on NotificationCallback type alias and get_notification_sender method
- handlers/mod.rs: #[allow(dead_code)] on past_complete_month helper
- Cargo.toml: move maintenance key into [package.metadata] (was invalid at package level)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Cargo.toml

@@ -11,6 +11,8 @@ keywords = ["mt5", "mql5", "trading", "mcp", "backtest"]
 categories = ["finance", "development-tools"]
 homepage = "https://github.com/masdevid/mt5-quant"
 documentation = "https://github.com/masdevid/mt5-quant"
+
+[package.metadata]
 maintenance = { status = "actively-developed" }
 
 [[bin]]