fix(acceptance): 校准 Seedance 图片转换验收

按 Volces 官方输入边界补齐 Seedance 2.0 候选能力，将错误的合法 4K 转换样本替换为真实越界图片，并让协议模拟器校验物化后的 Base64 data URL。\n\n验证：Go 全量测试、迁移安全检查、gofmt。

apps/api/cmd/acceptance-load/main.go

@@ -848,7 +848,7 @@ func (o options) emulatorFixtureURLs() []string {
 		out = append(out, fmt.Sprintf("%s/fixtures/image-%02d.webp", o.emulatorURL, index))
 	}
 	for index := 12; index < 16; index++ {
-		out = append(out, fmt.Sprintf("%s/fixtures/image-%02d-4k.jpg", o.emulatorURL, index))
+		out = append(out, fmt.Sprintf("%s/fixtures/image-%02d-oversized.jpg", o.emulatorURL, index))
 	}
 	return out
 }

apps/api/cmd/migrate/main_test.go

@@ -52,6 +52,28 @@ func TestSeedanceInputImageConstraintMigrationKeepsCatalogSnapshotsInSync(t *tes
 	}
 }
 
+func TestVolcesSeedanceInputImageConstraintMigrationUsesDocumentedBounds(t *testing.T) {
+	payload, err := os.ReadFile("../../migrations/0098_volces_seedance20_input_image_constraints.sql")
+	if err != nil {
+		t.Fatal(err)
+	}
+	content := string(payload)
+	for _, required := range []string{
+		"volces:doubao-seedance-2-0-260128",
+		"volces:doubao-seedance-2-0-fast-260128",
+		"volces:doubao-seedance-2-0-mini-260615",
+		`"long_edge":300`,
+		`"long_edge":6000`,
+		`'[0.4,2.5]'::jsonb`,
+		"'{metadata,rawModel,capabilities}'",
+		"UPDATE platform_models",
+	} {
+		if !strings.Contains(content, required) {
+			t.Fatalf("Volces Seedance input image migration is missing %q", required)
+		}
+	}
+}
+
 func TestSecurityEventSchemaMigrationsDefineCurrentLifecycle(t *testing.T) {
 	streamPayload, err := os.ReadFile("../../migrations/0063_oidc_security_events.sql")
 	if err != nil {