feat(region_pricing): 扩展非 token 统一计费字段，支持语音按字符/秒计费

- 新增 region_pricing.pricing_mode / price_unit / flat_price 字段
- 新增 migration 016_region_pricing_non_token_units.sql
- officialPricingRecord 新增 PricingMode/PriceUnit/FlatPrice 字段
- detectModality 新增 audio 模态检测（voice/audio/speech）
- providerMetadata 新增 BAAI/ByteDance/China Mobile 元数据
- import_mobile_cloud_pricing.go: 解析语音计费表（CosyVoice/SenseVoice）
  - CosyVoice: 2元/万字符 → pricingMode=flat, priceUnit=10k_characters
  - SenseVoice: 0.0007元/秒 → pricingMode=flat, priceUnit=second
- mobileCloudProviderName 新增 cosyvoice/sensevoice → Alibaba 映射
- cmd/server: modelResponse 新增 pricingMode/priceUnit/flatPrice，API 字段说明同步更新
- 新增 TestModelsHandlerReturnsFlatPricingFields 测试

cmd/server/main_test.go

@@ -10,6 +10,55 @@ import (
 	"testing"
 )
 
+func TestModelsHandlerReturnsFlatPricingFields(t *testing.T) {
+	mux := newMux(
+		&sql.DB{},
+		func(context.Context, *sql.DB) ([]modelResponse, error) {
+			return []modelResponse{{
+				ID:             "mobile-cloud-huabei-huhehaote-cosyvoice",
+				Name:           "CosyVoice",
+				Provider:       "Alibaba",
+				ProviderCN:     "阿里云",
+				Modality:       "audio",
+				PricingMode:    "flat",
+				PriceUnit:      "10k_characters",
+				FlatPrice:      2,
+				Currency:       "CNY",
+				IsFree:         false,
+				DataConfidence: "official",
+			}}, nil
+		},
+		func(context.Context, *sql.DB) ([]subscriptionPlanResponse, error) {
+			return nil, nil
+		},
+		func(context.Context, *sql.DB) (*latestReportResponse, error) {
+			return nil, sql.ErrNoRows
+		},
+	)
+
+	req := httptest.NewRequest(http.MethodGet, "/api/v1/models", nil)
+	rec := httptest.NewRecorder()
+	mux.ServeHTTP(rec, req)
+
+	if rec.Code != http.StatusOK {
+		t.Fatalf("expected status 200, got %d", rec.Code)
+	}
+
+	var payload struct {
+		Data []modelResponse `json:"data"`
+	}
+	if err := json.Unmarshal(rec.Body.Bytes(), &payload); err != nil {
+		t.Fatalf("unmarshal response: %v", err)
+	}
+	if len(payload.Data) != 1 {
+		t.Fatalf("expected 1 model, got %d", len(payload.Data))
+	}
+	got := payload.Data[0]
+	if got.PricingMode != "flat" || got.PriceUnit != "10k_characters" || got.FlatPrice != 2 {
+		t.Fatalf("unexpected flat pricing payload: %+v", got)
+	}
+}
+
 func TestSubscriptionPlansHandlerReturnsEnvelope(t *testing.T) {
 	mux := newMux(
 		&sql.DB{},