fix: keep ai_xiaomu avatar versioned

[GaryLauLGY]

Summary

• add the verified 400×400 versioned 黄小木 avatar asset
• keep frontend bootstrap and server migration references on the versioned key
• keep the material regeneration source aligned so future syncs do not restore the cached avatar path

Verification

• pnpm test (2/2)
• pnpm lint
• pnpm data:migrate:dry (329 materials, 4 accounts, 10 backgrounds, 5 avatars; no duplicate IDs or invalid links)
• production publishing account and source author already reference /api/v1/media/avatars/ai-xiaomu-2076716603392323584.png
• public media SHA-256 matches the local asset: 07a3aabedd4cdef438d39d8539a378cf4f15a04da7b83a6ff9b7e83f12de1d08