Trigger reviews from requested reviewers

[haasonsaas]

Summary

• Add configurable GitHub pull_request review trigger events for the server webhook path.
• Gate review_requested automation by requested reviewer login so EvalOpsBot can be used as an on-demand deep-review reviewer without reviewing every PR.
• Wire the new settings through Helm and self-hosting docs.

Test plan

• cargo fmt --check
• cargo test review_request
• cargo test github_auto_review
• cargo test opened_action_matches_when_enabled_without_reviewer_gate
• cargo clippy --all-targets -- -D warnings
• helm template diffscope charts/diffscope --set diffscope.github.autoReviewEvents='{review_requested}' --set diffscope.github.reviewRequestReviewers='{EvalOpsBot}'
• pre-push: workflow validation, version sync, fmt, clippy, web build/test, full cargo test

Closes

N/A