feat: remove nebula

[earayu]

Note

Removes NebulaGraph across code, deployments, tests, and docs, standardizing graph storage on Neo4j/PG.

• Backend:
  • Delete aperag/db/nebula_sync_manager.py and aperag/graph/lightrag/kg/nebula_sync_impl.py.
  • Remove NebulaSyncStorage import/registry from aperag/graph/lightrag/kg/__init__.py.
• Deploy/Config:
  • Strip Nebula envs/values from Helm templates (aperag-secret.yaml, api/celery/flower deployments) and deploy/aperag/values.yaml.
  • Remove Nebula vars from envs/env.template and envs/docker.env.overrides.
  • Delete nebula-docker-compose.yml.
• Tests:
  • Remove Nebula E2E tests and env templates under tests/e2e_test/graphstorage.
• Docs:
  • Update architecture and graph index docs (EN/ZH and web) to drop NebulaGraph mentions and examples; keep Neo4j/PG only.

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[apecloud-bot]

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[apecloud-bot]

This branch name is not following the standards: feature/|bugfix/|release/|hotfix/|support/|releasing/|dependabot/

[apecloud-bot]

This branch name is not following the standards: feature/|bugfix/|release/|hotfix/|support/|releasing/|dependabot/