Fix Y2038 bug for JWT on 32 bit plattforms

Ensure the expire field is always 64-bit to prevent issues in 2038
on 32-bit platforms, where `usize` is only 32 bits wide.

src/auth/jwt.rs

@@ -17,7 +17,7 @@ const JWT_ALGORITHM: Algorithm = Algorithm::HS512;
 #[derive(Debug, Serialize, Deserialize)]
 pub struct UserClaims {
     pub pid: String,
-    exp: usize,
+    exp: u64,
     claims: Option<Value>,
 }
 
@@ -70,8 +70,7 @@ impl JWT {
         pid: String,
         claims: Option<Value>,
     ) -> JWTResult<String> {
-        #[allow(clippy::cast_possible_truncation)]
-        let exp = (get_current_timestamp() + expiration) as usize;
+        let exp = get_current_timestamp().saturating_add(*expiration);
 
         let claims = UserClaims { pid, exp, claims };