fix(webhook): fill source metadata and persist ReviewOutput for webhook reviews (#137)

[Liewzheng]

Fixes #137.

What changed

• Webhook-triggered reviews (GitLab/GitHub) now build a rich "SourceMeta" from the incoming payload and back-fill authoritative fields from MRInfo before the expert run, so the History list no longer shows "Untitled" / "unknown" for MR title, project, or author.
• The task store now persists the full "ReviewOutput" (including per-expert reports) when a webhook review completes, so the History detail panel renders the expert results instead of an empty list.
• GitHub webhook handler is wired into the task store lifecycle (previously only GitLab recorded tasks).
• Serialization failure during outcome recording now marks the task Failed instead of Completed with a null result.
• GitHub PR payload now extracts target_branch from pull_request.base.ref.

Verification

• cargo fmt --check ✅
• cargo clippy --all-targets --all-features ✅
• cargo test ✅ (1571 passed)
• cd frontend && npm run build ✅