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/codes_lib_migrate_test.go

@@ -104,6 +104,12 @@ func TestCodesLibMigrateRoundTrip(t *testing.T) {
 	// the reviewer's repro hit: the lib's `codes` table collides with the
 	// name 000020's down script renames legacy_codes back to.
 	m := newSQLiteStepper(t, db)
+	// 000021 (pay_purchases/source-enum) now sits on top of 000020 and is
+	// unrelated to this collision — step it back down first so we land
+	// exactly on the 000020 boundary the test targets.
+	if err := m.Steps(-1); err != nil {
+		t.Fatalf("step 000021 down: %v", err)
+	}
 	if err := m.Steps(-1); err != nil {
 		t.Fatalf("step 000020 down: %v (this is the reviewer-reported collision — "+
 			"000020 down must DROP the codes-lib tables before renaming legacy_* back)", err)

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(