feat: gravity transaction logging + user-facing history popup

- New GravityTransaction schema tracks all gravity changes (registration,
  interview/optimize/download deduction, purchase, monthly topup, migration)
- GravityTopUpService: bulk log for monthly VIP topup
- PaymentController.activateMembership: log plan_set transactions
- QuotaService: add logTransaction, wire into all gravity-modifying methods
- UserService: log registration grants (phone/wx/email/password)
- GET /user/gravity-transactions?page=&limit= API
- Frontend: user.vue '明细' button + paginated popup
- Docs: update PROJECT-STATUS v4.10, FEATURE-LIST, DEPLOYMENT

backend/src/modules/schemas/gravity-transaction.schema.ts

@@ -0,0 +1,43 @@
+import { Prop, Schema, SchemaFactory } from '@nestjs/mongoose'
+import { Document } from 'mongoose'
+
+export type GravityTransactionDocument = GravityTransaction & Document
+
+export type GravityTransactionType =
+  | 'registration'           // 注册赠送
+  | 'interview_deduct'       // AI 面试消耗
+  | 'optimize_deduct'        // 简历优化消耗
+  | 'download_deduct'        // 简历下载消耗
+  | 'purchase'               // 充值购买
+  | 'monthly_topup'          // 月度补给
+  | 'migration'              // 旧额度迁移
+  | 'plan_set'               // 套餐设置
+  | 'share'                  // 分享所得
+  | 'contribution'           // 面经贡献奖励
+  | 'share_credits_fallback' // 分享币后备抵扣
+  | 'admin_adjust'           // 管理员调整
+
+@Schema({ timestamps: true })
+export class GravityTransaction {
+  @Prop({ required: true, index: true })
+  userId: string
+
+  @Prop({ required: true })
+  amount: number       // 变动数量（正=增加，负=减少）
+
+  @Prop({ required: true })
+  balance: number      // 变动后余额
+
+  @Prop({ required: true })
+  type: GravityTransactionType
+
+  @Prop({ default: '' })
+  description: string  // 描述（如 "AI 模拟面试消耗"）
+
+  @Prop()
+  refId?: string       // 关联 ID（订单号、面试ID 等）
+}
+
+export const GravityTransactionSchema = SchemaFactory.createForClass(GravityTransaction)
+
+GravityTransactionSchema.index({ userId: 1, createdAt: -1 })