feat(safety): add Phase C advanced controls

- Add rollback manager with file checkpointing and transaction context
- Add HITL manager with approval queues and notification handlers
- Add content filter with PII, secrets, and injection detection
- Add emergency controls with stop/pause/resume capabilities
- Update SafetyConfig with checkpoint_dir setting

Issue #63

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

backend/app/services/safety/config.py

@@ -74,6 +74,10 @@ class SafetyConfig(BaseSettings):
 
     # Rollback settings
     rollback_enabled: bool = Field(True, description="Enable rollback capability")
+    checkpoint_dir: str = Field(
+        "/tmp/syndarix_checkpoints",  # noqa: S108
+        description="Directory for checkpoint storage",
+    )
     checkpoint_retention_hours: int = Field(24, description="Checkpoint retention")
     auto_checkpoint_destructive: bool = Field(
         True, description="Auto-checkpoint destructive actions"