Upgrade default search to GRAPH_COMPLETION

[krapfj23]

Summary

Switch the default search from INSIGHTS (raw triplets) to GRAPH_COMPLETION (vector + graph + LLM reasoning).

Blocked by

• Research: GRAPH_COMPLETION vs INSIGHTS result comparison #77 (GRAPH_COMPLETION vs INSIGHTS research)

Blocks

• Expose multiple SearchType options via query param #79 (expose multiple SearchType options)

Steps

1. Open backend/app/services/knowledge_search.py
2. Change the default SearchType from INSIGHTS to GRAPH_COMPLETION
3. Update the search response model if GRAPH_COMPLETION returns a different format (it returns natural language answers vs raw triplets)
4. Update tests to expect the new format

Acceptance Criteria

• GET /documents/search?q=... returns LLM-synthesized answers with graph context
• Response model handles GRAPH_COMPLETION format
• Tests updated and passing
• PR body includes Closes #78