feat(server): 迁移 000021 pay_purchases 台账 + subscriptions.source 扩 pay

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_013nMthbVEmQquxBRKb9Fj8u

server/internal/store/sqlite_migrate_test.go

@@ -29,8 +29,8 @@ func TestSQLiteMigrateUpDown(t *testing.T) {
 	if dirty {
 		t.Fatalf("schema dirty after MigrateUp")
 	}
-	if v != 20 {
-		t.Errorf("version = %d, want 20", v)
+	if v != 21 {
+		t.Errorf("version = %d, want 21", v)
 	}
 
 	// 2. Core tables exist.
@@ -39,6 +39,7 @@ func TestSQLiteMigrateUpDown(t *testing.T) {
 		"usage_daily", "audit_log", "providers", "nodes", "node_events",
 		"directory_version", "provision_idempotency", "replacements", "admins",
 		"connect_credentials", "usage_device_daily", "sessions", "usage_hourly", "usage_device_hourly",
+		"pay_purchases",
 	} {
 		var name string
 		err := db.QueryRow(