feat(ops): move Slack management to ops

Summary

• Move Slack workspace management from account settings to an admin-only /ops/slack page with dedicated ops navigation and route styles.
• Gate Slack install, callback, and disconnect actions behind the stored instance admin role.
• Redirect /ops to /ops/slack and preserve the originally requested local path through login.
• Update Slack self-hosting docs, OpenGraph coverage, and focused route/auth/layout tests.

Testing

• mix format
• mix test
• mix compile --warnings-as-errors
• git diff --check

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