feat(graph): [2/3] node population and edge detection

Objective

After each scan, populate the graph with resource nodes from the
InventorySnapshot and infer edges between them with evidence provenance.

Depends on: #331

Scope

Node population (scanner/graph/node_service.py)

• Upsert graph_nodes from snapshot.resources keyed on (tenant_id, resource_id)
• Link each finding to its node via finding_graph_nodes
• Run as a post-scan step; failure logs but does not fail the scan

Edge detection (scanner/graph/edge_detector.py)

• EdgeDetector base class: detect(snapshot, nodes) -> list[GraphEdge]
• Initial detectors:
  • NsgToSubnet (PROTECTS)
  • SubnetToResource (MEMBER_OF)
  • PublicIpToResource (EXPOSES)
  • IdentityToResource (HAS_IDENTITY)
  • StoragePrivateEndpoint (REACHABLE_VIA)
• Confidence: ARG-confirmed 1.0, inferred from properties 0.8,
  cross-referenced 0.6
• Upsert edges on each scan; no duplicates

Test plan

• Mock snapshot produces correct nodes, no duplicates on re-scan
• All findings link to a node after scan
• Each detector tested with fixture snapshots
• No cross-tenant nodes or edges