Fix similar_to vector query ordering for large libraries

[vswamidass-sfdc]

Summary

• remove updated_at ordering from Document.related_by_embedding so nearest-neighbor distance ordering remains index-friendly
• use reorder('neighbor_distance ASC') in the similar_to path so prior scope ordering cannot force expensive re-sorting
• keep semantic ranking distance-first for large library queries

Test plan

• Manual rails console validation for library_id=327 showed near-instant response with distance-only query shape
• Verified changed files are lint-clean

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