#247 docs: account_id override for DELETE /api/artists/{id} and GET /api/chats/{id}/messages

Contract for the admin account-override work tracked in recoupable/chat#1811. Documents the optional account_id override on the first two flagged endpoints so the docs land before the API implementations.

What changed

DELETE /api/artists/{id} (releases.json)

• Adds an optional account_id request body (UUID) — lets a caller with multi-account access (org members / Recoup admins) delete an artist in another account's context.
• Adds a 400 response for an invalid account_id UUID.
• Updates the endpoint description.

GET /api/chats/{id}/messages (research.json)

• Adds an optional account_id query param (camelCase accountId alias also accepted) — lets a caller read another account's chat messages.
• Broadens the 400 description to cover an invalid account_id.
• Updates the endpoint description.

Both mirror the existing override docs on POST /api/artists (body) and GET /api/artists (query). Diffs are additive; both JSON specs validate.

Merge order

This docs PR lands first, then the two API PRs:

1. 📄 docs (this PR) → main
2. ⚙️ DELETE /api/artists/{id} override → api test
3. ⚙️ GET /api/chats/{id}/messages override → api test

Tracking issue: recoupable/chat#1811

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Summary by cubic

Adds docs for an optional account_id override on DELETE /api/artists/{id} so org/admin users can delete an artist in another account’s context. Also documents a 400 error for an invalid account_id UUID.

• New Features
  • DELETE /api/artists/{id}: optional account_id in JSON body; 400 for invalid UUID.