DIP: Decouple the Database Layer

Problem

UserService directly instantiates MySQLDatabase inside its constructor: `this.db = new MySQLDatabase()`. This means switching to PostgreSQL requires editing UserService. Testing UserService requires a real database. Apply Dependency Inversion — high-level modules should not depend on low-level modules; both should depend on abstractions.

Requirements

IUserRepository interface with: findById(id), save(user), delete(id)
MySQLUserRepository implements IUserRepository
PostgreSQLUserRepository implements IUserRepository (same contract)
UserService accepts IUserRepository in its constructor (constructor injection)
Show how to swap database without touching UserService
Show how to inject a MockUserRepository in tests

Constraints

–UserService must not import or reference MySQLUserRepository or PostgreSQLUserRepository
–All dependencies must be injected — no new inside UserService

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