- Add `folder_limit` field to account model
- Support limiting sync per folder/label for both IMAP and Gmail API accounts
- Ensures each folder/label syncs at most `folder_limit` messages
- Added REST endpoint `POST /append-reply-to-draft/:account_id` that creates a reply draft linked to the original email thread with appropriate headers (`References`, `In-Reply-To`).
- Added equivalent gRPC method for creating and appending reply drafts.