tidying stuff up, dto for search

common/migrations/20251210_add_embedding_backend_to_system_settings.surql

@@ -0,0 +1,8 @@
+-- Add embedding_backend field to system_settings for visibility of active backend
+
+DEFINE FIELD IF NOT EXISTS embedding_backend ON system_settings TYPE option<string>;
+
+-- Set default to 'openai' for existing installs to preserve backward compatibility
+UPDATE system_settings:current SET
+    embedding_backend = 'openai'
+WHERE embedding_backend == NONE;