chore: pet art pipeline checkpoint + Hashiden rebrand

[SunshineRider]

What & why

Checkpoint of the pet-art pipeline work plus the Hashiden rebrand for the content engine, on the shared feat/mainnet-readiness-state-loop branch used across the mainnet-readiness effort.

Changes

• 41a4c57 chore: checkpoint pre-existing pet art pipeline work (build + full test suite green at checkpoint)
• e64141b chore: hashiden rebrand sweep and docs cleanup — retired MineBTC-era naming removed; one legacy asset-domain env value flagged for infra (domain cutover, not code)

Verification

• npm run build (tsc) green; test suite green at checkpoint.

Notes

• Out of scope for this PR per the mainnet-readiness plan: NFT image/artwork pipeline changes (pet art generation is triggered by backend mints; the backend side of that loop was E2E-verified on localnet — job dispatch, BullMQ job-id format fix, and wedged-row recovery are in the hashiden-backend PR).