Migrate graph store config: Kuzu → Neo4j Aura

[krapfj23]

Summary

Update cognee_config.py to support Neo4j Aura as the graph store.

Blocked by

• Research: Neo4j Aura setup + free tier limits #65 (Neo4j Aura research)

Blocks

• Re-ingest test data on Neo4j + verify graph queries #67 (re-ingest + verify on Neo4j)

Steps

1. Update backend/requirements.txt: add neo4j driver, add cognee[neo4j] if not already using it
2. Update backend/app/cognee_config.py: add Neo4j config block reading GRAPH_DATABASE_PROVIDER, GRAPH_DATABASE_URL, GRAPH_DATABASE_USERNAME, GRAPH_DATABASE_PASSWORD
3. Update .env.example with new Neo4j vars
4. Update the configure() call to pass graph store settings
5. Remove or gate the Kuzu-specific volume/path configuration (keep it as a fallback behind GRAPH_DATABASE_PROVIDER check)

Acceptance Criteria

• App starts with Neo4j config vars set
• cognee.cognify() writes to Neo4j Aura
• Kuzu still works as fallback when GRAPH_DATABASE_PROVIDER is not set to neo4j
• PR body includes Closes #66