Add optional email verification and registration control

Implemented email service infrastructure with SMTP support and optional email verification for self-hosted deployments. Registration flow now supports open/closed modes and hardcoded Apocapoc branding.

Key features:
- Email service with SMTP and template rendering
- Optional email verification (auto-verified without SMTP config)
- Registration modes: open/closed for access control
- Hardcoded Apocapoc branding (AppName, AppURL, DefaultFrom)
- Separate registration and login flows (registration no longer returns tokens)

internal/domain/repositories/user_repository.go

@@ -10,5 +10,6 @@ type UserRepository interface {
 	Create(ctx context.Context, user *entities.User) error
 	FindByID(ctx context.Context, id string) (*entities.User, error)
 	FindByEmail(ctx context.Context, email string) (*entities.User, error)
+	FindByVerificationToken(ctx context.Context, token string) (*entities.User, error)
 	Update(ctx context.Context, user *entities.User) error
 }