refactor(store): DB 规约统一 —— 雪花字符串 id + created/updated/deleted_at 软删

所有数据库映射结构体收敛到统一基类，清理混乱。

- store/base.go：BaseModel{ID string(雪花,bwmarrin/snowflake) PK, CreatedAt, UpdatedAt,
  DeletedAt gorm软删index} + BeforeCreate 生成 id + NewID()。
- 全模型嵌入 BaseModel：User/Task/LLMModel/KB/Doc/Agent（去掉各自 ID uint/CreatedAt）；
  Task 业务 id(task_xxx)挪到 TaskID 唯一列，主键统一雪花。
- 模型 id uint→string：admin :id 路由、SetActiveModel/DeleteModel/SaveModel、modelBody.ID。
- 一次性迁移 migrateLegacyIntIDs：检测旧整型 id（AutoMigrate 不改主键类型）→ 备份
  sundynix_model 行（唯一不可再生的 API 密钥）→ 删旧表 → 按新规约重建 → 回灌模型（新雪花 id）。
  其它表(User/Task/KB/Doc/Agent)为可重建测试数据，重置。
- Doc 预埋 Size/Preview/ObjectKey 字段，DocLink 表（为后续 B/C）。

验证：重启 gateway → 日志"回灌 2 条模型配置"；PG sundynix_model.id=varchar、有
created_at/updated_at/deleted_at；DeepSeek/百炼 密钥保留(keylen 35)；admin 列表返回
雪花 string id + 脱敏 key；健康五灯全绿。gateway build 通过。

注：mcp-go 的 sundynix_user_profile(Profile) 模型尚未套同规约，待跟进对齐。

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

sundynix-gateway/go.sum

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