fix: sync MCP stats model and scripts with voyage-4-lite
- expand Stats type with embeddings, searches, errors, started_at,
cache_hits, cache_misses, and per-endpoint timing metrics
- update backfill-embeddings to use voyage-4-lite with output_dimension
- update rebuild-documents-table to use F32_BLOB(1024)
- add regression tests for full Stats model
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>