#638 Priority-based rate limiting for tools

Implement priority parameter for rate limiting to allow critical operations to bypass or have higher limits. Priority Levels: - low: marketing, bulk (most restrictive) - medium: informational (default) - high: important notifications - critical: password reset, security alerts (bypass/unlimited) Use Cases: - Password reset emails must never be blocked - Test emails from admin should bypass limits - Marketing campaigns get strict limits - Security alerts always go through Implementation: 1. Add priority_limits JSONB column to tool_rate_limits 2. Rate limiter accepts priority parameter 3. Tool decorator extracts _priority from kwargs 4. API endpoints to configure priority limits 5. Critical priority can bypass rate limiting entirely Related: Issue #637 (time-based capacity)