Remove user field from chat DB model and update API docs

[Saby-Bishops]

Motivation

• The chat document interface no longer requires a per-document user property, so the model was simplified to reflect the actual stored schema.
• API request examples and payload docs referenced a user field that isn't part of the chat payload, so documentation was updated to avoid confusion.
• Risk: removing the field may break external consumers that expect user on CouchDB chat documents, so consumer expectations should be confirmed before rollout.

Description

• Removed the user: any; property from the DbDoc interface in chatapi/src/models/db-doc.model.ts.
• Updated the chat API request examples to omit user from the POST payload and websocket request samples in chatapi/README.md.
• Adjusted the additional data description in the README to remove the user entry so examples and docs match the model.

Testing

• No automated tests were run for this change.

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Codex Task