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

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Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

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

@@ -0,0 +1,19 @@
+"""Safety metrics collection and export."""
+
+from .collector import (
+    MetricType,
+    MetricValue,
+    SafetyMetrics,
+    get_safety_metrics,
+    record_mcp_call,
+    record_validation,
+)
+
+__all__ = [
+    "MetricType",
+    "MetricValue",
+    "SafetyMetrics",
+    "get_safety_metrics",
+    "record_mcp_call",
+    "record_validation",
+]