Feature: Slack integration

[aparajon]

Add a Slack integration that allows for the following:

• Posts schema change events to a dedicated Slack channel that is specified in the server config
• Notifies users in a Slack thread of schema change events
• Shows a link to a Slack support channel that will show up in PR comments

[aparajon]

Implementation note: ProgressObserver pattern

PR #109 introduces a ProgressObserver interface in pkg/tern/observer.go that receives notifications from the apply progress poller:

type ProgressObserver interface {
    OnProgress(apply *storage.Apply, tasks []*storage.Task)
    OnTerminal(apply *storage.Apply, tasks []*storage.Task)
}

This pattern is designed to support multiple notification backends. The first implementation is CommentObserver (posts GitHub PR comments), but the same interface can be used for Slack:

observer := MultiObserver{
    NewCommentObserver(...),         // PR comments
    NewSlackObserver(channel, ...),  // Slack channel notifications
}

A SlackObserver would:

• Post to a configured Slack channel on apply start, progress milestones, and terminal state
• Thread replies for per-table progress updates
• Include links back to the PR and apply ID

The observer is set per-apply by whoever initiates it — the webhook handler, CLI, or server config default. It runs on the same goroutine as the schema change execution, so it survives container restarts (the recovery worker reconstructs the observer from the apply record's stored context).

Key design constraint: observer methods must be fire-and-forget. If Slack is down, the schema change keeps running. Errors are logged but never block execution.

This comment was generated by Claude Opus 4.6 (1M context).