Convert the /chats/all endpoint from loading all user chats into memory
at once to a streaming NDJSON response that fetches chats in batches of
100. This prevents Out-of-Memory crashes for users with large chat
histories.
Backend: Added async generator that paginates through chats with
short-lived DB sessions per batch (critical for SQLite lock release).
Frontend: Updated getAllChats to consume the NDJSON stream via
ReadableStream reader, accumulating results for the export file.
Ref: open-webui#22206