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

backend/migrations/086_channel_platform_pricing.sql

@@ -0,0 +1,21 @@
+-- 086_channel_platform_pricing.sql
+-- 渠道按平台维度：model_pricing 加 platform 列，model_mapping 改为嵌套格式
+
+-- 1. channel_model_pricing 加 platform 列
+ALTER TABLE channel_model_pricing
+    ADD COLUMN IF NOT EXISTS platform VARCHAR(50) NOT NULL DEFAULT 'anthropic';
+
+CREATE INDEX IF NOT EXISTS idx_channel_model_pricing_platform
+    ON channel_model_pricing (platform);
+
+-- 2. model_mapping: 从扁平 {"src":"dst"} 迁移为嵌套 {"anthropic":{"src":"dst"}}
+-- 仅迁移非空、非 '{}' 的旧格式数据（通过检查第一个 value 是否为字符串来判断是否为旧格式）
+UPDATE channels
+SET model_mapping = jsonb_build_object('anthropic', model_mapping)
+WHERE model_mapping IS NOT NULL
+  AND model_mapping::text NOT IN ('{}', 'null', '')
+  AND NOT EXISTS (
+      SELECT 1 FROM jsonb_each(model_mapping) AS kv
+      WHERE jsonb_typeof(kv.value) = 'object'
+      LIMIT 1
+  );