#69 API: List Available Permissions

Implement endpoint to list all available permissions. Endpoint: GET /api/v1/permissions Response: List of all permissions with descriptions Required Permissions: Any authenticated user (read-only) Implementation Requirements: - Return all 27 permissions - Group by category (user, workflow, execution, admin) - Include description for each - Include which roles have each permission Permission Categories: - User Management: user:create, user:read, user:update, user:delete - Role Management: role:assign, role:revoke - Workflow: workflow:create, workflow:read, workflow:update, workflow:delete, workflow:execute - Execution: execution:read, execution:cancel, execution:retry - Events: event:read, event:emit - Schedules: schedule:create, schedule:read, schedule:update, schedule:delete - Admin: admin:audit, admin:settings, admin:tokens, admin:api_keys, admin:invitations - System: system:tenant_manage, system:system_settings With ultra attention to details and correctness. Before closing: write comment summarizing implementation with screenshot.