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

- 缓存重构为 O(1) 哈希结构 (pricingByGroupModel, mappingByGroupModel)
- 渠道模型映射接入网关流程 (Forward 前应用, a→b→c 映射链)
- 新增 billing_model_source 配置 (请求模型/最终模型计费)
- usage_logs 新增 channel_id, model_mapping_chain, billing_tier 字段
- 每种计费模式统一支持默认价格 + 区间定价
- 渠道模型限制开关 (restrict_models)
- 分组按平台分类展示 + 彩色图标
- 必填字段红色星号 + 模型映射 UI
- 去除模型通配符支持

backend/internal/service/channel_test.go

@@ -15,7 +15,6 @@ func TestGetModelPricing(t *testing.T) {
 	ch := &Channel{
 		ModelPricing: []ChannelModelPricing{
 			{ID: 1, Models: []string{"claude-sonnet-4"}, BillingMode: BillingModeToken, InputPrice: channelTestPtrFloat64(3e-6)},
-			{ID: 2, Models: []string{"claude-*"}, BillingMode: BillingModeToken, InputPrice: channelTestPtrFloat64(5e-6)},
 			{ID: 3, Models: []string{"gpt-5.1"}, BillingMode: BillingModePerRequest},
 		},
 	}
@@ -28,9 +27,8 @@ func TestGetModelPricing(t *testing.T) {
 	}{
 		{"exact match", "claude-sonnet-4", 1, false},
 		{"case insensitive", "Claude-Sonnet-4", 1, false},
-		{"wildcard match", "claude-opus-4-20250514", 2, false},
-		{"exact takes priority over wildcard", "claude-sonnet-4", 1, false},
 		{"not found", "gemini-3.1-pro", 0, true},
+		{"wildcard pattern not matched", "claude-opus-4-20250514", 0, true},
 		{"per_request model", "gpt-5.1", 3, false},
 	}