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

db/migrations/016_region_pricing_non_token_units.sql

@@ -0,0 +1,31 @@
+-- Phase 2: region_pricing 扩展非 token 统一计费字段（字符/秒等）
+
+ALTER TABLE region_pricing
+    ADD COLUMN IF NOT EXISTS pricing_mode TEXT NOT NULL DEFAULT 'input_output',
+    ADD COLUMN IF NOT EXISTS price_unit TEXT NOT NULL DEFAULT 'million_tokens',
+    ADD COLUMN IF NOT EXISTS flat_price REAL;
+
+DO $$
+BEGIN
+    IF NOT EXISTS (
+        SELECT 1
+        FROM pg_constraint
+        WHERE conname = 'chk_region_pricing_pricing_mode'
+    ) THEN
+        ALTER TABLE region_pricing
+            ADD CONSTRAINT chk_region_pricing_pricing_mode
+            CHECK (pricing_mode IN ('input_output', 'flat'));
+    END IF;
+END
+$$;
+
+UPDATE region_pricing
+SET pricing_mode = 'input_output'
+WHERE coalesce(pricing_mode, '') = '';
+
+UPDATE region_pricing
+SET price_unit = 'million_tokens'
+WHERE coalesce(price_unit, '') = '';
+
+CREATE INDEX IF NOT EXISTS idx_region_pricing_pricing_mode ON region_pricing(pricing_mode);
+CREATE INDEX IF NOT EXISTS idx_region_pricing_price_unit ON region_pricing(price_unit);