feat: server-side BFS entity graph endpoint #82

Summary

Add a server-side BFS graph traversal endpoint for visualizing entity relationships in the ontology editor.

Requirements

• GET /projects/{id}/ontology/classes/{iri}/graph endpoint
• Configurable ancestors_depth, descendants_depth, max_nodes, include_see_also
• EntityGraphResponse schema with GraphNode and GraphEdge models
• Lineage-based node types: focus, root, class, individual, property, external, unexplored
• Truncation flag when node count exceeds max_nodes

Companion

Frontend: CatholicOS/ontokit-web#88