perf: Increase Wikidata HNSW ef_construction from 384 to 512

Bump ef_construction for 50M vector scale indexing to improve index quality
and graph connectivity. Higher ef_construction builds a denser, higher-quality
graph at index creation time, enabling better recall during search.

For 768-dimensional vectors at 50M scale, the higher construction factor
better compensates for curse of dimensionality and should improve recall
from current levels.

Changes:
- ef_construction: 384 → 512 (16.7% increase)
- Wikidata HNSW config now: m=32, ef_construction=512, ef_search_default=400

Author: shisoft

src/apps/wikidata/schema.rs

@@ -218,7 +218,7 @@ pub async fn ensure_wikidata_embedding_indexes(server: &Arc<MorpheusServer>) ->
 
     let wikidata_hnsw_config = neb::index::vector::HnswConfig {
         m: 32,
-        ef_construction: 384,
+        ef_construction: 512,
         ef_search_default: 400,
     };