[MeshCore P3] M4 - Neo4j export to PATH_OBSERVED

[pskillen]

Parent

Part of #267. Milestone M4.

Goal

Graph backend for topology + centrality, isolated from Meshtastic traceroute.

Scope

• Export MeshCorePathEdgeBucket → new PATH_OBSERVED relationship (separate from ROUTED_TO) with protocol, evidence, weight, resolved, bucket props.
• Generalized node key (mc:<internal_id> / mc_hash:<size>:<hash>); do not reuse the MT integer node_id.
• Async + idempotent from rollups only (never per-observation); manage.py bulk export.

Acceptance criteria

• Re-export is idempotent (MERGE on stable key).
• MT ROUTED_TO graph + heatmap untouched.

Decisions (locked)

• New PATH_OBSERVED relationship type.