- 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.
- Add two new APIs to list email threads and fetch all messages within a thread
- Create a new email thread table to store thread information
- Implement method to extract thread_id from emails to support thread association queries
- Added unified search interface to query messages by email address
- Implemented both OpenAPI and gRPC endpoints
- Introduced new index table to optimize email-based lookups
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