feat(integrations): add Slack OAuth integration and file listing support

Summary

• Add Slack Bot OAuth integration via n8n workflow
• Add file listing capability for cloud providers (Google Drive, SharePoint, S3)
• Prepare Teams integration webhook configuration

Changes

Slack Integration

• SlackService (services/slack_service.py): Connection, listing, and disconnection of Slack accounts
• Endpoints:
  • POST /integrations/oauth/slack/connect - Connect Slack via bot token
  • GET /integrations/oauth/slack/accounts - List user's Slack accounts
  • DELETE /integrations/oauth/slack/{account_uid} - Disconnect account

File Listing

• FileItem schema for file metadata (id, name, mimeType, size, dates)
• FileListResponse schema for API response
• list_files() method in FolderService

Configuration

• Added Slack/Teams webhook paths in oauth_service.py

Test plan

• Test Slack connect with valid token → 200 OK
• Test Slack connect with missing token → 400
• Test Slack connect with invalid format → 400
• Test without x-user-uuid header → 401
• Test list accounts → 200 OK
• Test disconnect non-existent → 404
• Test disconnect existing → 200 OK