feat(deps): add modbus, database, mocking, and async trait dependencies

Adds tokio-modbus 0.17.0 for Modbus RTU over TCP communication,
sqlx with runtime-tokio and sqlite features for async database operations,
mockall 0.13 for trait mocking in tests, and async-trait 0.1 for async trait support.

Ref: T001 (specs/001-modbus-relay-control)

nix/shell.nix

@@ -26,6 +26,7 @@ inputs.devenv.lib.mkShell {
         cargo-tarpaulin
         just
         marksman # Markdown LSP server
+        sqlx-cli
         tombi # TOML LSP server
       ];