feat: add local auth billing and usage management

tests/seedance-client.test.ts

@@ -0,0 +1,10 @@
+import { describe, expect, it } from "vitest";
+import { extractSeedanceUsage } from "@/lib/seedance/client";
+
+describe("Seedance usage extraction", () => {
+  it("reads completion_tokens from top-level and nested provider responses", () => {
+    expect(extractSeedanceUsage({ usage: { completion_tokens: 12345 } })).toEqual({ completionTokens: 12345 });
+    expect(extractSeedanceUsage({ data: { usage: { completionTokens: 67890 } } })).toEqual({ completionTokens: 67890 });
+    expect(extractSeedanceUsage({ usage: { prompt_tokens: 100 } })).toBeUndefined();
+  });
+});