fix: replace unstable is_multiple_of and update Cargo.lock for sha2

The Docker image uses rust:1.83-slim where is_multiple_of is unstable.
Also regenerates Cargo.lock to include the sha2 dependency.

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

Cargo.lock

@@ -1510,6 +1510,17 @@ dependencies = [
  "digest",
 ]
 
+[[package]]
+name = "sha2"
+version = "0.10.9"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "a7507d819769d01a365ab707794a4084392c824f54a7a6a7862f8c3d0892b283"
+dependencies = [
+ "cfg-if",
+ "cpufeatures",
+ "digest",
+]
+
 [[package]]
 name = "sharded-slab"
 version = "0.1.7"
@@ -2491,6 +2502,7 @@ dependencies = [
  "rusqlite",
  "serde",
  "serde_json",
+ "sha2",
  "shellexpand",
  "tempfile",
  "thiserror 2.0.18",

src/security/secrets.rs

@@ -241,7 +241,7 @@ fn hex_encode(data: &[u8]) -> String {
 
 /// Hex-decode a hex string to bytes.
 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())