Add Slack integration for JIT access approval workflows

[haasonsaas]

Context

Gate has the JIT access request data model (access_requests table with pending/approved/denied/expired states) but no notification path. Approvals require calling the API directly, making the feature nearly unusable in practice. Formal.ai ships native Slack and Jira approval workflows.

Proposal

Build a Slack bot integration for JIT access approvals:

• When an access request is created, post a message to a configurable Slack channel with:
  • Requester identity, requested resource, requested duration, reason
  • Approve / Deny buttons (Slack interactive message)
• Button clicks call the Gate control plane API to approve/deny
• Requester gets a DM notification when approved/denied
• Configurable: approval channel per resource or per org
• Optional: thread notifications for request expiry

Acceptance Criteria

• Access request triggers Slack notification with approve/deny buttons
• Button clicks update request status via Gate API
• Requester notified on approval/denial
• Configurable approval channels
• Documentation for Slack app setup and OAuth scopes

[haasonsaas]

Shipped the webhook-contract foundation in #104.

What landed:

• richer payloads for access.requested, access.approved, and access.denied
• README documentation for those access-request webhook events
• server coverage for request/approve/deny webhook payloads

That means Gate already has the outbound event path a Slack bridge can consume. The remaining work on this issue is Slack-specific: inbound interactive actions, approval-channel routing, and requester notifications.