fix: propagate SMTP transport errors from EmailService.sendEmail

sendEmail caught transport failures internally and resolved to false or
undefined, so every caller's .catch() was dead code and send failures were
silently suppressed. Password recovery in particular reported "email sent
successfully" while the user received nothing.

sendEmail now throws an AppError carrying the underlying cause, and returns
Promise<string> instead of Promise<string | false | undefined>.

Call sites updated for the new contract:
- UserService.registerUser: the existing .catch() now actually fires
- UserService.requestRecovery: the failure reaches the client instead of a
  misleading success response
- InviteService and the test-email endpoint: keep their user-facing messages
  and attach the SMTP cause
- EmailProvider: catches and returns false, because NotificationsService fans
  providers out through Promise.all and a rejection there would abort the
  other channels' alerts

Closes #3757

server/src/api/controllers/settingsController.ts

@@ -93,24 +93,30 @@ class SettingsController implements ISettingsController {
 		if (!html) {
 			throw new AppError({ message: "Failed to build email template.", status: 500 });
 		}
-		const messageId = await this.emailService.sendEmail(to, subject, html, {
-			systemEmailHost,
-			systemEmailPort,
-			systemEmailUser,
-			systemEmailAddress,
-			systemEmailDisplayName,
-			systemEmailPassword,
-			systemEmailConnectionHost,
-			systemEmailSecure,
-			systemEmailPool,
-			systemEmailIgnoreTLS,
-			systemEmailRequireTLS,
-			systemEmailRejectUnauthorized,
-			systemEmailTLSServername,
-		});
-
-		if (!messageId) {
-			throw new AppError({ message: "Failed to send test email.", status: 500 });
+		let messageId: string;
+		try {
+			messageId = await this.emailService.sendEmail(to, subject, html, {
+				systemEmailHost,
+				systemEmailPort,
+				systemEmailUser,
+				systemEmailAddress,
+				systemEmailDisplayName,
+				systemEmailPassword,
+				systemEmailConnectionHost,
+				systemEmailSecure,
+				systemEmailPool,
+				systemEmailIgnoreTLS,
+				systemEmailRequireTLS,
+				systemEmailRejectUnauthorized,
+				systemEmailTLSServername,
+			});
+		} catch (error: unknown) {
+			// Surface the underlying SMTP failure: diagnosing the settings is the whole
+			// point of the test email endpoint.
+			throw new AppError({
+				message: error instanceof Error ? `Failed to send test email. ${error.message}` : "Failed to send test email.",
+				status: 500,
+			});
 		}
 
 		return res.status(200).json({