feat(challenges): 支持自定义挑战类型并优化必填字段验证

- 新增 CUSTOM 挑战类型枚举值
- requirementLabel 字段改为可选，允许为空并添加默认值处理
- minimumCheckInDays 最大值从 365 提升至 1000，支持更长周期挑战
- 推送通知模板支持自定义挑战的动态文案生成
- 新增 getCustomEncouragementTemplate 和 getCustomInvitationTemplate 函数

src/challenges/challenges.service.ts

@@ -164,7 +164,7 @@ export class ChallengesService {
         image: challenge.image,
         periodLabel: challenge.periodLabel,
         durationLabel: challenge.durationLabel,
-        requirementLabel: challenge.requirementLabel,
+        requirementLabel: challenge.requirementLabel || '',
         status,
         unit: challenge.progressUnit,
         startAt: new Date(challenge.startAt).getTime(),
@@ -278,7 +278,7 @@ export class ChallengesService {
       image: challenge.image,
       periodLabel: challenge.periodLabel,
       durationLabel: challenge.durationLabel,
-      requirementLabel: challenge.requirementLabel,
+      requirementLabel: challenge.requirementLabel || '',
       summary: challenge.summary,
       rankingDescription: challenge.rankingDescription,
       highlightTitle: challenge.highlightTitle,
@@ -1205,7 +1205,7 @@ export class ChallengesService {
       endAt: new Date(challenge.endAt).getTime(),
       periodLabel: challenge.periodLabel,
       durationLabel: challenge.durationLabel,
-      requirementLabel: challenge.requirementLabel,
+      requirementLabel: challenge.requirementLabel || '',
       summary: challenge.summary,
       targetValue: challenge.targetValue,
       progressUnit: challenge.progressUnit,