Implements three-tier retrieval system to address scalability and precision
limitations in production environments with large document collections.
Features:
- Tier 1: Knowledge Base Routing (auto/rule-based/llm-based)
- Tier 2: Document Filtering (metadata-based)
- Tier 3: Chunk Refinement (vector search with parent-child support)
Changes:
- Add HierarchicalRetrieval class with configurable retrieval pipeline
- Add hierarchical_retrieval_config field to Dialog model
- Add database migration for new configuration field
- Add comprehensive unit tests (35 tests, all passing)
Fixes#11610
