[codex] Add GitHub App backend event relay

[Daliys]

Summary

Adds an event-driven GitHub App backend path so CI Scope can consume GitHub Actions state from webhooks and an outbound SSE stream instead of constantly polling GitHub from the Mac.

What changed

• Added a small FastAPI/SQLite backend under backend/ with webhook signature verification, normalized workflow run/job storage, snapshot reads, and SSE streaming.
• Updated the local broker to prefer backend snapshots/SSE and use slow GitHub polling only as fallback.
• Mirrored backend snapshots into the existing local broker state files so the Swift UI contract stays intact.
• Added Swift-side caching and broker-feed-first workflow run loading to avoid GitHub calls for fresh empty state.
• Added LaunchAgent pass-through for backend URL/token environment settings.
• Documented backend setup and added backend/broker tests.

Validation

• python3 -m py_compile backend/ci_scope_backend/*.py 'CI Scope/Broker/CI Scope Broker'
• python3 -m unittest discover tests
• xcodebuild -project 'CI Scope.xcodeproj' -scheme 'CI Scope' -configuration Debug -destination 'platform=macOS' build

Notes

The requested dev branch does not exist on the remote, so this PR targets develop.