[Epic 953][Stage I][chat-api] Retain owner-only Conversation views and block writes

[think-in-universe]

Parent epic: nearai/cloud-api#953
Parent stage: nearai/cloud-api#954
Parent component: #385
Target date: Sep 1

Goal

Retain only the existing owner-scoped Conversation read views needed for migration/export while blocking every normal Conversation proxy write.

Retained temporary views

The following existing, owner-scoped, session-authenticated views remain available with Cache-Control: no-store:

• GET /v1/conversations
• GET /v1/conversations/{conversation_id}
• GET /v1/conversations/{conversation_id}/items

The chat-api list is an existing owner-scoped view used to identify a user's records. It is not a new export API or a new public Cloud Conversation-list endpoint. Shared and optional-auth/public Conversation reads are outside the temporary contract.

Disabled operations

Conversation creation, update, deletion, item creation, and any unsupported or unlisted legacy Conversation path return authenticated 410 Gone with Cache-Control: no-store. Disabled operations must not make a Cloud mutation or change local Conversation state.

This disables the session-authenticated Chat proxy route only. It does not remove Cloud #943's direct, workspace-scoped Conversation resource DELETE retained for the #380 account-deletion worker.

All sharing routes, including GET /v1/conversations/{conversation_id}/shares, are closed under #389. Pin/archive/clone are also tracked there.

Acceptance criteria

• The three views above remain owner-scoped, session-authenticated, and work against Cloud's retained read surface.
• POST /v1/conversations, write/delete methods on a Conversation, POST /items, and unknown legacy descendants are 410/no-store.
• OpenAPI/docs list only the temporary owner Conversation views.