Replace Neo4j GDS with in-process graphology for PageRank and community detection

AuraDB Free/Standard blocks access to the GDS (Graph Data Science) plugin, which is required for Leiden community detection and PageRank algorithms. Without GDS, the nightly pipeline falls back to weakly connected components (WCC) for community detection and degree counting for PageRank — both produce significantly worse results than the real algorithms.

WCC just finds disconnected subgraphs (no modularity optimization), so communities are coarse and don't capture thematic clusters. Degree counting assigns scores by connection count rather than graph centrality, so landmark promotion is unreliable. ClusterConcept nodes computed from WCC communities are meaningless for search diversification.