chore: harden common storage bootstrap and slim embedded db assets

Unify embedding config, build providers from system settings, and fail
startup when index builds error or time out. Move Surreal assets under
common/db so embeds exclude crate source, and read storage via streams.

common/migrations/20251210_add_embedding_backend_to_system_settings.surql

@@ -1,8 +0,0 @@
--- 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;