[FEAT] Add Slack integration for channel notifications

[martian56]

Type / Priority

Feature / Medium

Summary

There is no Slack integration for posting notifications to channels.

Current state (evidence)

• model/integration.go mentions "slack" only in a comment; no Slack model, OAuth provider, handler, or service; ui/src/components/integrations/ contains only GitHub components. (GitHub integration is complete.)

Proposed scope

Add a per-project Slack channel/token model, Slack OAuth + install handler, a notification sender, a channel-select UI, and instance settings for Slack app credentials.