Allow null steps in AchievementUpdateRequest

Jackson 3 (brought in by Spring Boot 4) defaults
FAIL_ON_NULL_FOR_PRIMITIVES to true and treats absent JSON properties
as null for primitive record components. The producer omits steps for
REVEAL/UNLOCK operations, which Jackson 2 silently coerced to 0 but
Jackson 3 now rejects, turning every such message into a poison pill
that blocks the achievement queue.

Switch steps to Integer and document via @Schema that it is only
required for INCREMENT and SET_STEPS_AT_LEAST.

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

Author: Brutus5000

src/main/java/com/faforever/api/achievements/AchievementUpdateRequest.java

@@ -1,10 +1,13 @@
 package com.faforever.api.achievements;
 
+import io.swagger.v3.oas.annotations.media.Schema;
+
 record AchievementUpdateRequest(
   int playerId,
   String achievementId,
   Operation operation,
-  int steps
+  @Schema(description = "Required for INCREMENT and SET_STEPS_AT_LEAST; ignored (and may be null/omitted) for REVEAL and UNLOCK.", nullable = true)
+  Integer steps
 ) {
   public enum Operation {
     REVEAL, UNLOCK, INCREMENT, SET_STEPS_AT_LEAST