feat(safety): add Phase D MCP integration and metrics

- Add MCPSafetyWrapper for safe MCP tool execution
- Add MCPToolCall/MCPToolResult models for MCP interactions
- Add SafeToolExecutor context manager
- Add SafetyMetrics collector with Prometheus export support
- Track validations, approvals, rate limits, budgets, and more
- Support for counters, gauges, and histograms

Issue #63

backend/app/services/safety/mcp/__init__.py

@@ -0,0 +1,17 @@
+"""MCP safety integration."""
+
+from .integration import (
+    MCPSafetyWrapper,
+    MCPToolCall,
+    MCPToolResult,
+    SafeToolExecutor,
+    create_mcp_wrapper,
+)
+
+__all__ = [
+    "MCPSafetyWrapper",
+    "MCPToolCall",
+    "MCPToolResult",
+    "SafeToolExecutor",
+    "create_mcp_wrapper",
+]