fix(be): 海报 pet_avatar_url 一直为空——ownedPet 不解析头像

GetPoster 用 ownedPet 只填了 Age，头像 URL 没解析，导致 pet_avatar_url
恒为空、海报总是退回 emoji。改成显式 s.fileURL(pet.AvatarFileID)。

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

pets-be/internal/service/report.go

@@ -229,8 +229,9 @@ func (s *Service) GetPoster(userID, petID string) (*Poster, error) {
 	s.db.Model(&model.HealthRecord{}).Where("pet_id = ? AND type = ? AND occurred_at >= ?", petID, model.RecordVaccine, weekAgo).Count(&vaccineRecs)
 	s.db.Model(&model.HealthRecord{}).Where("pet_id = ? AND type = ? AND category = ? AND occurred_at >= ?", petID, model.RecordSymptom, "高", weekAgo).Count(&highRisk)
 
+	// ownedPet 只填了 Age，头像 URL 得在这里从 file id 解析（否则是空的）
 	return &Poster{
-		PetName: pet.Name, PetEmoji: pet.Emoji, PetAvatarURL: pet.AvatarURL,
+		PetName: pet.Name, PetEmoji: pet.Emoji, PetAvatarURL: s.fileURL(pet.AvatarFileID),
 		Age: pet.Age, Weight: pet.Weight, Stage: pet.Stage,
 		Period:         weekAgo.Format("01.02") + " - " + now.Format("01.02"),
 		TasksCompleted: tasksDone, WeightRecords: weightRecs, VaccineRecords: vaccineRecs,