feat: Add Neo4j + GraphQL POC integration

[priya-madraslabs]

• Add Neo4j client with graceful fallback (EXPERIMENTAL)
• Add GraphQL schema and HTTP server (read-only)
• Integrate Neo4j lookups in entity extraction and text unit retrieval
• Feature flag controlled via GRAPHRAG_USE_NEO4J
• Fully backward compatible with fallback to dataframe logic

Files:

• 4 new files in graphrag/graph/ (Neo4j client, GraphQL schema/server)
• 2 modified workflow files (entity_extraction.py, text_units.py)
• 1 documentation file (neo4j_graphql_poc.md)

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