feat(challenges): 新增挑战功能模块及完整接口实现

- 新增挑战列表、详情、加入/退出、进度上报等 REST 接口
- 定义 Challenge / ChallengeParticipant 数据模型与状态枚举
- 提供排行榜查询与用户排名计算
- 包含接口文档与数据库初始化脚本

src/challenges/models/challenge-participant.model.ts

@@ -0,0 +1,99 @@
+import {
+  Table,
+  Column,
+  DataType,
+  Model,
+  ForeignKey,
+  BelongsTo,
+  Index,
+} from 'sequelize-typescript';
+import { Challenge } from './challenge.model';
+import { User } from '../../users/models/user.model';
+
+export enum ChallengeParticipantStatus {
+  ACTIVE = 'active',
+  COMPLETED = 'completed',
+  LEFT = 'left',
+}
+
+@Table({
+  tableName: 't_challenge_participants',
+  underscored: true,
+})
+export class ChallengeParticipant extends Model {
+  @Column({
+    type: DataType.CHAR(36),
+    defaultValue: DataType.UUIDV4,
+    primaryKey: true,
+  })
+  declare id: string;
+
+
+  @ForeignKey(() => Challenge)
+  @Column({
+    type: DataType.CHAR(36),
+    allowNull: false,
+    comment: '挑战 ID',
+  })
+  declare challengeId: string;
+
+
+  @ForeignKey(() => User)
+  @Column({
+    type: DataType.STRING(64),
+    allowNull: false,
+    comment: '用户 ID',
+  })
+  declare userId: string;
+
+  @Column({
+    type: DataType.INTEGER,
+    allowNull: false,
+    defaultValue: 0,
+    comment: '当前进度值',
+  })
+  declare progressValue: number;
+
+  @Column({
+    type: DataType.INTEGER,
+    allowNull: false,
+    comment: '目标值，通常与挑战 targetValue 相同',
+  })
+  declare targetValue: number;
+
+  @Column({
+    type: DataType.ENUM('active', 'completed', 'left'),
+    allowNull: false,
+    defaultValue: ChallengeParticipantStatus.ACTIVE,
+    comment: '参与状态',
+  })
+  declare status: ChallengeParticipantStatus;
+
+  @Column({
+    type: DataType.DATE,
+    allowNull: false,
+    defaultValue: DataType.NOW,
+    comment: '加入时间',
+  })
+  declare joinedAt: Date;
+
+  @Column({
+    type: DataType.DATE,
+    allowNull: true,
+    comment: '退出时间',
+  })
+  declare leftAt: Date | null;
+
+  @Column({
+    type: DataType.DATE,
+    allowNull: true,
+    comment: '最近一次更新进度的时间',
+  })
+  declare lastProgressAt: Date | null;
+
+  @BelongsTo(() => Challenge)
+  declare challenge?: Challenge;
+
+  @BelongsTo(() => User)
+  declare user?: User;
+}