feat: enable audit logging for authentication and task operations

Add audit_service.log_event() calls to track key user activities:
- auth_login: successful and failed login attempts with IP/user agent
- auth_logout: single session and all sessions logout
- task_delete: task deletion with user context
- file_upload: file upload with filename, size, and type
- admin_cleanup: manual cleanup trigger with statistics

Each event captures client IP (from X-Forwarded-For/X-Real-IP headers),
user agent, and relevant metadata for compliance and debugging.

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

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

backend/app/routers/admin.py

@@ -405,6 +405,22 @@ async def trigger_cleanup(
             f"{result['total_files_deleted']} files, {result['total_bytes_freed']} bytes"
         )
 
+        # Log admin cleanup action
+        audit_service.log_event(
+            db=db,
+            event_type="admin_cleanup",
+            event_category="admin",
+            description=f"Manual cleanup: {result['total_files_deleted']} files, {result['total_bytes_freed']} bytes freed",
+            user_id=admin_user.id,
+            success=True,
+            metadata={
+                "files_deleted": result['total_files_deleted'],
+                "bytes_freed": result['total_bytes_freed'],
+                "users_processed": result['users_processed'],
+                "max_files_per_user": files_to_keep
+            }
+        )
+
         return {
             "success": True,
             "message": "Cleanup completed successfully",