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/entities/user.go

@@ -3,12 +3,15 @@ package entities
 import "time"
 
 type User struct {
-	ID           string
-	Email        string
-	PasswordHash string
-	Timezone     string
-	CreatedAt    time.Time
-	UpdatedAt    time.Time
+	ID                      string
+	Email                   string
+	PasswordHash            string
+	Timezone                string
+	EmailVerified           bool
+	EmailVerificationToken  *string
+	EmailVerificationExpiry *time.Time
+	CreatedAt               time.Time
+	UpdatedAt               time.Time
 }
 
 func NewUser(email, passwordHash, timezone string) *User {