feat: add schema drift handling and rebuild

backend/db/schema.sql

@@ -111,6 +111,8 @@ CREATE TABLE IF NOT EXISTS settings (
     value_enc TEXT
 );
 
+CREATE TABLE IF NOT EXISTS schema_version (version INTEGER PRIMARY KEY);
+
 -- 7. prompts + prompt_revisions (append-only revisions)
 CREATE TABLE IF NOT EXISTS prompts (
     id INTEGER PRIMARY KEY,