feat(auth): add signIn rate-limit backfill with dual ip/account budgets

Wires the rate-limit primitive end-to-end through auth.signIn as the
canonical credential-stuffing defence example:

- manifest: rateLimit [ip 5/1m, account 10/1h] on signIn use case
- use case: rateLimit: IRateLimit dep; dual consume + TooManyRequestsError
- binders: ctx.rateLimit ?? new NoopRateLimit() in bind-production + bind-dev-seed
- tRPC: TooManyRequestsError → TOO_MANY_REQUESTS error code in authProcedure
- tests: RecordingRateLimit dual-consume assertion; InMemoryRateLimit
  budget-1 ip + account rejection; coverage 100% on use-cases layer
- ESLint: _manifest-ast.js extractRateLimitNames handles RateLimitBudget
  objects ({name,window,budget}) in addition to plain string literals,
  no-undeclared-rate-limit passes on both "ip" and "account" call sites

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

packages/auth/src/entities/errors/auth.ts

@@ -18,3 +18,10 @@ export class UnauthorizedError extends Error {
     this.name = "UnauthorizedError";
   }
 }
+
+export class TooManyRequestsError extends Error {
+  constructor(message: string, options?: ErrorOptions) {
+    super(message, options);
+    this.name = "TooManyRequestsError";
+  }
+}

packages/auth/src/entities/errors/errors.test.ts

@@ -3,6 +3,7 @@ import {
   AuthenticationError,
   UnauthenticatedError,
   UnauthorizedError,
+  TooManyRequestsError,
 } from "./auth";
 import { InputParseError } from "./common";
 
@@ -37,3 +38,12 @@ describe("InputParseError", () => {
     expect(err.message).toBe("invalid input");
   });
 });
+
+describe("TooManyRequestsError", () => {
+  it("is an instance of Error with the given message", () => {
+    const err = new TooManyRequestsError("too many attempts");
+    expect(err).toBeInstanceOf(Error);
+    expect(err.message).toBe("too many attempts");
+    expect(err.name).toBe("TooManyRequestsError");
+  });
+});