feat(auth): T18 implement user registration endpoint

Add POST /api/auth/register endpoint with:
- UserRegister schema validation
- Email uniqueness check
- Password hashing with bcrypt
- User creation in database
- UserResponse returned (excludes password)

Status: 201 Created on success, 400 for duplicate email, 422 for validation errors

Test coverage: 5 tests for register endpoint

src/openrouter_monitor/routers/__init__.py

@@ -0,0 +1,4 @@
+"""Routers package for OpenRouter Monitor."""
+from openrouter_monitor.routers import auth
+
+__all__ = ["auth"]