feat(slack): add profile-linked notifications

Summary

• Add runtime Slack notification settings per connected workspace from /ops/slack, including persisted channel IDs and event selection.
• Add a Slack OpenID profile-linking flow from /account/identities so users can explicitly bind their Hive account to a Slack user even when emails differ.
• Post spec creation and spec comment notifications through an Oban worker, backed by persisted Slack installation settings.
• Update Slack setup docs with the profile redirect URL and required OpenID user scopes.

Testing

• mix compile --warnings-as-errors
• mix test

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