test: add integration tests for RepoIndexer (Neo4j + Qdrant)

[vineethwilson15]

Goal

Integration tests for core/indexer/ against real Neo4j + Qdrant containers.

Acceptance Criteria

• tests/integration/test_indexer_integration.py marked @pytest.mark.integration
• Index a small synthetic repo
• Assert File node in Neo4j + vector in Qdrant
• Teardown cleans up test data

Good First Issue

Unit tests and indexer are already in place -- just add real DB assertions.