feat(alerts): include deployment integration metadata in alerts

Add resolution and propagation of deployment integration metadata (git
links and Vercel deployment URL) to alert delivery flow so emails,
Slack messages and webhooks can include richer context for deployment
alerts.

- Resolve Git metadata and Vercel integration info via
  BranchesPresenter and Vercel integration schema in a new
  #resolveDeploymentMetadata flow; represent result as
  DeploymentIntegrationMetadata.
- Thread DeploymentIntegrationMetadata through DeliverAlertService and
  pass it into #sendEmail, #sendSlack and #sendWebhook calls.
- Populate email payload with git details (branch, short SHA, commit
  message and PR info) and vercelDeploymentUrl when present.
- Use neverthrow.fromPromise to safely handle metadata resolution and
  fall back to empty metadata on errors.

Motivation: provide recipients immediate links and SCM context for
deployment success/failure alerts to speed debugging and navigation.

Author: 0ski