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__/dead-token-verification-removal.test.ts

@@ -80,17 +80,7 @@ describe("Dead Token Verification System Removal", () => {
       expect(typeof verification.verifyCode).toBe("function");
     });
 
-    it("should export isEmailVerified", async () => {
-      const verification = await import("../services/verification.js");
-      expect(verification).toHaveProperty("isEmailVerified");
-      expect(typeof verification.isEmailVerified).toBe("function");
-    });
-
-    it("should export getVerifiedApiKey", async () => {
-      const verification = await import("../services/verification.js");
-      expect(verification).toHaveProperty("getVerifiedApiKey");
-      expect(typeof verification.getVerifiedApiKey).toBe("function");
-    });
+    // isEmailVerified and getVerifiedApiKey removed — only used by dead signup router
 
     it("should export PendingVerification interface", async () => {
       // TypeScript interface test - if compilation passes, the interface exists