完善多赛道联调与全局产品架构

backend/migrations/0007_variant_minimal.sql

@@ -0,0 +1,11 @@
+BEGIN;
+
+ALTER TABLE game_sessions
+  ADD COLUMN assignment_mode TEXT CHECK (assignment_mode IN ('manual', 'random', 'server-assigned')),
+  ADD COLUMN variant_id TEXT,
+  ADD COLUMN variant_name TEXT;
+
+CREATE INDEX game_sessions_variant_id_idx ON game_sessions(variant_id);
+CREATE INDEX game_sessions_assignment_mode_idx ON game_sessions(assignment_mode);
+
+COMMIT;