### What problem does this PR solve?
Implement:
1. `/api/v1/datasets/<dataset_id>/documents/<document_id>/chunks GET`
2.
`/api/v1/datasets/<dataset_id>/documents/<document_id>/chunks/<chunk_id>
PATCH`
3. `/api/v1/datasets/<dataset_id>/documents/<document_id>/chunks PATCH`
### Type of change
- [x] New Feature (non-breaking change which adds functionality)
Implements POST /api/v1/searchbots/ask in Go with streaming SSE,
citations, and think-tag processing. 23 files, 90+ unit tests.
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Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>