feat: expose record parity APIs over REST

[dcfocus]

Summary

• Add API trait and remote client methods for external-id lookup, logical record delete, and related-record traversal.
• Expose REST routes for delete by id/external_id, get by external_id, and related records with lifecycle visibility options.
• Add focused server tests for the new handlers and router construction.

Closes #75.

Testing

• cargo fmt --all -- --check
• cargo test -p lance-context-server
• cargo test --workspace
• cargo clippy --workspace --all-targets -- -D warnings
• git diff --check