feat(channel): 缓存扁平化 + 网关映射集成 + 计费模式统一 + 模型限制

- 缓存按 (groupID, platform, model) 三维 key 扁平化，避免跨平台同名模型冲突
- buildCache 批量查询 group platform，按平台过滤展开定价和映射
- model_mapping 改为嵌套格式 {platform: {src: dst}}
- channel_model_pricing 新增 platform 列
- 前端按平台维度重构：每个平台独立配置分组/映射/定价
- 迁移 086: platform 列 + model_mapping 嵌套格式迁移

frontend/src/i18n/locales/en.ts

@@ -1806,7 +1806,13 @@ export default {
         restrictModels: 'Restrict Models',
         restrictModelsHint: 'When enabled, only models in the pricing list are allowed. Others will be rejected.',
         defaultPerRequestPrice: 'Default per-request price (fallback when no tier matches)',
-        defaultImagePrice: 'Default image price (fallback when no tier matches)'
+        defaultImagePrice: 'Default image price (fallback when no tier matches)',
+        platformConfig: 'Platform Configuration',
+        addPlatform: 'Add Platform',
+        noPlatforms: 'Click "Add Platform" to start configuring the channel',
+        mappingCount: 'mappings',
+        pricingEntry: 'Pricing Entry',
+        noModels: 'No models added'
       }
     },