feat(marketing): user profile update, ad placement management, logo limits

- Add PUT /auth/profile endpoint for nickname and avatar updates
- Add ad_placements table and CRUD admin API for managing ad units
- Add GET /marketing/ad-config public API for mini-program to fetch ad config
- Reduce logo limit from 10 to 3 per user, add 2MB file size validation

Made-with: Cursor

internal/marketing/model/ad_placement.go

@@ -0,0 +1,19 @@
+package model
+
+import "time"
+
+type AdPlacement struct {
+	ID            uint      `json:"id" gorm:"primaryKey;comment:主键ID"`
+	MiniProgramID uint      `json:"mini_program_id" gorm:"not null;index;comment:小程序ID"`
+	Name          string    `json:"name" gorm:"size:100;not null;comment:广告位名称"`
+	AdType        string    `json:"ad_type" gorm:"size:50;not null;default:rewarded_video;comment:广告类型(rewarded_video/banner/interstitial)"`
+	AdUnitID      string    `json:"ad_unit_id" gorm:"size:200;comment:微信广告单元ID"`
+	Status        int       `json:"status" gorm:"default:1;comment:状态(0禁用/1启用)"`
+	Description   string    `json:"description" gorm:"size:500;comment:备注说明"`
+	CreatedAt     time.Time `json:"created_at" gorm:"comment:创建时间"`
+	UpdatedAt     time.Time `json:"updated_at" gorm:"comment:更新时间"`
+}
+
+func (AdPlacement) TableName() string {
+	return "marketing_ad_placements"
+}