fix(security): use UTF-8 charset in RateLimitFilter.sha256Short (SpotBugs DM_DEFAULT_ENCODING)

SpotBugs flagged String.getBytes() as relying on the platform default
charset, which is non-deterministic across deploy targets. Switch to
StandardCharsets.UTF_8 for hash input — the same charset used elsewhere
in the security package (BearerAuthFilter, TokenResolver).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: aksOps

src/main/java/io/github/randomcodespace/iq/config/security/RateLimitFilter.java

@@ -16,6 +16,7 @@
 import org.springframework.web.filter.OncePerRequestFilter;
 
 import java.io.IOException;
+import java.nio.charset.StandardCharsets;
 import java.security.MessageDigest;
 import java.security.NoSuchAlgorithmException;
 import java.time.Duration;
@@ -155,7 +156,8 @@ static String clientKey(HttpServletRequest request) {
     /** First 16 hex chars of SHA-256(input) — enough collision resistance for keying. */
     private static String sha256Short(String input) {
         try {
-            byte[] hash = MessageDigest.getInstance("SHA-256").digest(input.getBytes());
+            byte[] hash = MessageDigest.getInstance("SHA-256")
+                    .digest(input.getBytes(StandardCharsets.UTF_8));
             return HexFormat.of().formatHex(hash, 0, 8);
         } catch (NoSuchAlgorithmException e) {
             throw new IllegalStateException("SHA-256 unavailable", e);