GitHub PR and preview sync

## Goal

Central Station should automatically associate GitHub pull requests, preview URLs, and PR lifecycle events with the right tickets so the PM view stays current without manual updates.

## Desired behavior

- Link opened PRs to existing tickets when the PR title, branch name, body, or labels reference a Central Station/Yawp PM item.
- Create or flag placeholder tickets when an open PR has no PM association.
- Show PR links and preview/deploy URLs directly on ticket detail pages.
- Update ticket metadata when PRs open, close, merge, or change preview status.
- Prefer webhooks for near-real-time updates; keep an hourly sync job as a backstop.

## Open product questions

- Should webhook handling be GitHub App based or token/webhook-secret based for v1?
- What is the canonical preview URL source for Preview Forge and future preview systems?
- Should merged PRs automatically move tickets to needs-qa, ready-to-ship, or shipped?