Lock tool sessions to their model

migrations/20260901223000_add_session_model/down.sql

@@ -0,0 +1 @@
+ALTER TABLE sessions DROP COLUMN model;

migrations/20260901223000_add_session_model/up.sql

@@ -0,0 +1,7 @@
+ALTER TABLE sessions ADD COLUMN model TEXT
+    CHECK (model IS NULL OR model IN (
+        'deepseek-v4-flash-0731',
+        'deepseek-v4-pro',
+        'glm-5.2',
+        'glm-5.3-flash'
+    ));