[Skeyelab]

evaluateStuck and the stuck rules move out of the poller into
alerts.Watcher: Observe(repos, now) returns the alerts that fire this
cycle. It sees only Snapshots and the clock, so its tests drop the
webhook server and hand-built Poller.

Also fixes, while the policy moved:

• collapsing a Repo no longer reads as a PR recovering, so it no longer
  restarts the stuck clock
• webhooks are sent in the background; a slow endpoint used to block
  the poll cycle for up to 10s
• webhook failures log through logx instead of log.Printf, which wrote
  over the full-screen dashboard

Closes #53

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