feat: task-store integration for webhook reviews and queue monitor fix (v0.9.48)

[Liewzheng]

Summary

• Webhook-triggered GitLab MR reviews now create task-store entries, broadcast SSE events, and show up on the Queue Monitor / Recent Reviews.
• System hook verification falls back to the single configured platform when the payload carries no URL or no URL match (handles GitLab Test button).
• Queue Monitor gains a 'Recently completed' section and no longer blanks out when only completed tasks are present.
• Version bumped to 0.9.48.

Verification

• cargo fmt --check ✅
• cargo clippy --all-targets --all-features ✅
• cargo test ✅ (46 + 4 doc-tests)
• cd frontend && npm run build ✅
• End-to-end webbridge verification (real Chrome against local GitLab EE 19.2.4):
  • GitLab System Hook Test → reng /webhook/gitlab ✅ HTTP 200
  • reng review → GitLab MR note ✅
  • Dashboard / History / Queue pages ✅
  • Concurrent 4-MR run ✅

Notes

• AGENTS.md integration-testing rules already cover bidirectional webbridge testing and mandatory cleanup.
• The follow-up issue (webhook-created tasks missing MR title/project/author in source_meta because parse_mr_hook_payload only extracts URL+SHA today) will be filed separately as requested.