* perf: batch metadata query in KB retrieval to fix N+1 problem
Replace N sequential get_document_with_metadata() calls with a single
get_documents_with_metadata_batch() call using SQL IN clause.
Benchmark results (local SQLite):
- 10 docs: 10.67ms → 1.47ms (7.3x faster)
- 20 docs: 26.00ms → 2.68ms (9.7x faster)
- 50 docs: 63.87ms → 2.79ms (22.9x faster)
* refactor: use set[str] param type and chunk IN clause for SQLite safety
Address review feedback:
- Change doc_ids param from list[str] to set[str] to avoid unnecessary conversion
- Chunk IN clause into batches of 900 to stay under SQLite's 999 parameter limit
- Remove list() wrapping at call site, pass set directly
CAICAII
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