bmersereau puts a ceiling on runaway chat queries

Until now, asking the backend for a user's chat history meant getting the entire list - fine for a new account, less fine for a power user with hundreds of conversations. bmersereau's change caps the default response at the 50 most recent chats and lets callers page backward through older ones using a timestamp cursor, with a hard ceiling of 200 per request.

The move is quietly backward-compatible. Existing frontend code that asks for chats without specifying anything still works - it just gets a sensible slice instead of the whole pile. Alongside the feature, bmersereau wires up a proper test runner so the pagination behaviour is actually verified, not just assumed.