fix: replace unstable is_multiple_of with modulo for Rust 1.83 compat

The Docker image uses rust:1.83-slim where is_multiple_of is unstable.

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

src/security/secrets.rs

@@ -238,7 +238,7 @@ fn hex_encode(data: &[u8]) -> String {
 /// Hex-decode a hex string to bytes.
 #[allow(clippy::manual_is_multiple_of)]
 fn hex_decode(hex: &str) -> Result<Vec<u8>> {
-    if !hex.len().is_multiple_of(2) {
+    if hex.len() % 2 != 0 {
         anyhow::bail!("Hex string has odd length");
     }
     (0..hex.len())