Remove dead signup router, unused verification functions, and legacy cleanup query

- Delete src/routes/signup.ts (dead code, 410 handler in index.ts remains)
- Remove isEmailVerified() and getVerifiedApiKey() from verification.ts (only used by signup)
- Remove stale-key cleanup from cleanupStaleData() that queried legacy verifications table
- Update usage middleware message: 'Free tier limit' → 'Account limit'
- TDD: 8 new tests, removed signup.test.ts (dead), net 556 tests passing

src/__tests__/setup.ts

@@ -74,8 +74,7 @@ vi.mock("../services/browser.js", () => ({
 vi.mock("../services/verification.js", () => ({
   createPendingVerification: vi.fn().mockResolvedValue({ email: "test@test.com", code: "123456" }),
   verifyCode: vi.fn().mockResolvedValue({ status: "ok" }),
-  isEmailVerified: vi.fn().mockResolvedValue(false),
-  getVerifiedApiKey: vi.fn().mockResolvedValue(null),
+
 }));
 
 // Mock email service