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/infrastructure/http/auth_integration_test.go

@@ -22,15 +22,15 @@ func TestAuthFlow(t *testing.T) {
 			t.Errorf("Expected status 201, got %d. Body: %s", rr.Code, rr.Body.String())
 		}
 
-		var resp AuthResponse
+		var resp RegisterResponse
 		decodeResponse(t, rr, &resp)
 
-		if resp.Token == "" {
-			t.Error("Expected token in response")
-		}
 		if resp.UserID == "" {
 			t.Error("Expected user ID in response")
 		}
+		if resp.Message == "" {
+			t.Error("Expected message in response")
+		}
 	})
 
 	t.Run("Register duplicate email", func(t *testing.T) {