Share backtests as Community examples

Publish a completed backtest so other users can browse it and copy the setup.

• POST, PATCH, DELETE /api/v1/backtest/{job_id}/community-example publish, update or unpublish a listing (write:backtest).
• GET /api/v1/backtest/community-examples lists published examples.
• MCP: list_community_examples, get_community_example, publish_community_example, unpublish_community_example.