Get messages from multiple chats in bulk

Returns the most recent messages for up to 50 chats in a single request, keyed by the input chat UUID (the counterpart user UUID).

This is the bulk counterpart to GET /chats/{userUuid}/messages. It collapses N per-chat round trips into one POST and is intended for initial-sync flows and incremental refreshes.

Per-key errors are reported inside the 200 response body so a single unreachable chat never collapses the whole request — consumers keep the rest and only retry the failing keys.

Modes:

• Default: returns the latest limit messages per chat, newest first. hasMore indicates older messages exist.
• Incremental (sinceMessageUuid): returns messages strictly published after the cursor message's publish date, newest first.

Out of scope: Backwards history pagination (a before cursor). Use oldestMessageUuid from the response as the anchor when that ships.

<Note>Maximum 50 chat UUIDs per request. Failed keys are reported as { "error": "not_found" } — this covers both chats that don't exist and chats the caller isn't part of.</Note>

<Note>Read-only: fetching messages in bulk does not mutate read state — messages are not marked read and isRead reflects each message's state at read time.</Note>