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

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

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

@@ -1 +1,23 @@
-"""${dir} module."""
+"""Content filtering for safety."""
+
+from .filter import (
+    ContentCategory,
+    ContentFilter,
+    FilterAction,
+    FilterMatch,
+    FilterPattern,
+    FilterResult,
+    filter_content,
+    scan_for_secrets,
+)
+
+__all__ = [
+    "ContentCategory",
+    "ContentFilter",
+    "FilterAction",
+    "FilterMatch",
+    "FilterPattern",
+    "FilterResult",
+    "filter_content",
+    "scan_for_secrets",
+]