feat(channels): add custom account stats pricing rules

Allow channels to configure independent model pricing for account
statistics cost calculation, decoupled from user billing.

Backend:
- Migration 101: channels.apply_pricing_to_account_stats toggle,
  channel_account_stats_pricing_rules/model_pricing tables,
  usage_logs.account_stats_cost column
- resolveAccountStatsCost: match rules by group/account, then channel
  pricing, fallback to original formula when unconfigured
- Integrate into both GatewayService.recordUsageCore and
  OpenAIGatewayService.RecordUsage
- Update 8 account stats SQL queries to use
  COALESCE(account_stats_cost, total_cost) * account_rate_multiplier
- 23 unit tests for matching, pricing lookup, and cost calculation

Frontend:
- Channel edit dialog: toggle + custom rules UI with group/account
  multi-select and pricing entry cards
- API types and i18n (zh/en)

frontend/src/api/admin/channels.ts

@@ -34,6 +34,14 @@ export interface ChannelModelPricing {
   intervals: PricingInterval[]
 }
 
+export interface AccountStatsPricingRule {
+  id?: number
+  name: string
+  group_ids: number[]
+  account_ids: number[]
+  pricing: ChannelModelPricing[]
+}
+
 export interface Channel {
   id: number
   name: string
@@ -41,10 +49,11 @@ export interface Channel {
   status: string
   billing_model_source: string // "requested" | "upstream"
   restrict_models: boolean
-  features_config?: Record<string, unknown>
   group_ids: number[]
   model_pricing: ChannelModelPricing[]
   model_mapping: Record<string, Record<string, string>> // platform → {src→dst}
+  apply_pricing_to_account_stats: boolean
+  account_stats_pricing_rules: AccountStatsPricingRule[]
   created_at: string
   updated_at: string
 }
@@ -57,7 +66,8 @@ export interface CreateChannelRequest {
   model_mapping?: Record<string, Record<string, string>>
   billing_model_source?: string
   restrict_models?: boolean
-  features_config?: Record<string, unknown>
+  apply_pricing_to_account_stats?: boolean
+  account_stats_pricing_rules?: AccountStatsPricingRule[]
 }
 
 export interface UpdateChannelRequest {
@@ -69,7 +79,8 @@ export interface UpdateChannelRequest {
   model_mapping?: Record<string, Record<string, string>>
   billing_model_source?: string
   restrict_models?: boolean
-  features_config?: Record<string, unknown>
+  apply_pricing_to_account_stats?: boolean
+  account_stats_pricing_rules?: AccountStatsPricingRule[]
 }
 
 interface PaginatedResponse<T> {