Replace Neo4j temporal full scans with indexed temporal/vector retrieval

[ishaanxgupta]

neo4j_client.py computes semantic temporal similarity by scanning user event relationships with embeddings. This will not scale.

Acceptance criteria:

• Use Neo4j vector indexes or mirror temporal vectors into Pinecone
• Add date-range indexes and query paths.
• Benchmark temporal search at 10k, 100k, and 1M events per tenant.