Implement HiRAG, Sync, and Re-ranking Features

[google-labs-jules]

This submission implements three major new features as requested: HiRAG Phase 2 for automatic summarization and multi-hop retrieval, a cross-device synchronization backend with Firebase and WebSocket support, and advanced re-ranking strategies to improve search relevance. All necessary tests and documentation have been included.

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PR created automatically by Jules for task 8376656951597928097 started by @iberi22

[google-labs-jules]

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