add cargo-audit to Makefile lint target

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

Makefile

@@ -1,11 +1,11 @@
-.PHONY: all build lint fmt check test clean deploy
+.PHONY: all build lint fmt check audit test clean deploy
 
 all: lint build
 
 build:
 	cargo build
 
-lint: fmt check
+lint: fmt check audit
 
 fmt:
 	cargo fmt --check
@@ -13,6 +13,9 @@ fmt:
 check:
 	cargo clippy -- -D warnings
 
+audit:
+	cargo audit
+
 test:
 	cargo test