feat(billing): add metering formula admin tab

Expose margin multiplier, FX rate, and cost-mode settings under 计费中心 so operators can adjust DeepSeek billing without env edits.

Co-authored-by: Cursor <cursoragent@cursor.com>

src/types.ts

@@ -646,6 +646,27 @@ export type BlockedWord = {
   updated_at: number;
 };
 
+export type BillingAdminConfig = {
+  useBackendCost: boolean;
+  usdCnyRate: number;
+  marginMultiplier: number;
+  inputCentsPer1k: number;
+  outputCentsPer1k: number;
+  minBillCents: number;
+  costEstimateFromTokens: boolean;
+  costEstimateInputUsdPer1M: number;
+  costEstimateOutputUsdPer1M: number;
+};
+
+export type BillingAdminConfigResponse = {
+  config: BillingAdminConfig;
+  updatedAt: number | null;
+  updatedBy: string | null;
+  source?: string;
+  envOverrideActive?: boolean;
+  formula?: string;
+};
+
 export type PlanDefinition = {
   planType: string;
   name: string;