feat: add DeleteUser to storage interface and backends

[WilliamK112]

Summary

• Extend storage.Storage with DeleteUser(id string) error.
• Implement DeleteUser in both built-in backends (sqlite + postgres).
• Add storage tests for successful user deletion.
• Keep the simple in-memory service test storage implementation compliant with the expanded interface.
• Update README example of custom backend contract.

Closes #12

[pragneshbagary]

Good work on covering both backends and the README! One thing is missing though: internal/storage/memory/memory.go also needs DeleteUser InMemoryStorage no longer satisfies the Storage interface after this change.

Could you add it?

func (s *InMemoryStorage) DeleteUser(id string) error {
	s.mu.Lock()
	defer s.mu.Unlock()
	for username, user := range s.users {
		if user.ID == id {
			delete(s.users, username)
			return nil
		}
	}
	return nil
}

The reason the build passed locally is that nothing in this branch directly assigns *InMemoryStorage to storage.Storage, so the compiler didn't catch it but it will break as soon as someone uses the memory backend (e.g. in examples or tests).

[WilliamK112]

Thanks for catching that. I added \DeleteUser\ to \internal/storage/memory/memory.go\ and included a memory backend regression test with a compile-time \storage.Storage\ assertion in \internal/storage/memory/memory_test.go\.

I also ran \git diff --check\ locally. I couldn't run \go test ./...\ in this Codex environment because the Go toolchain isn't installed on PATH here.

[pragneshbagary]

Thanks!