Add Email Sending Service with Attachment Support (#3455)

# Description of Changes

Please provide a summary of the changes, including:

- **What was changed**  
- Introduced a new `EmailService` for asynchronous email delivery with
attachment support.
- Added `MailConfig` to configure a `JavaMailSender` bean using SMTP
settings from `ApplicationProperties`.
- Created `EmailController` endpoint (`/api/v1/general/send-email`) to
accept multipart/form-data requests for sending emails.
- Defined an `Email` API model to encapsulate recipient, subject, body,
and file input.
- Extended `ApplicationProperties` to include a nested `Mail` class for
SMTP host, port, username/password, and sender address.
- Updated `settings.yml.template` to include SMTP configuration
placeholders.
- Enhanced `.github/labeler-config.yml` to cover all new security- and
API-related source files for automated labeling.

- **Why the change was made**  
- To enable Stirling-PDF to notify users via email—particularly useful
for sending generated PDFs or alerts—directly from the application.
- To centralize mail server configuration in application properties and
streamline onboarding for new environments.

---

## Checklist

### General

- [x] I have read the [Contribution
Guidelines](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/CONTRIBUTING.md)
- [x] I have read the [Stirling-PDF Developer
Guide](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/DeveloperGuide.md)
(if applicable)
- [ ] I have read the [How to add new languages to
Stirling-PDF](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/HowToAddNewLanguage.md)
(if applicable)
- [x] I have performed a self-review of my own code
- [x] My changes generate no new warnings

### Documentation

- [ ] I have updated relevant docs on [Stirling-PDF's doc
repo](https://github.com/Stirling-Tools/Stirling-Tools.github.io/blob/main/docs/)
(if functionality has heavily changed)
- [ ] I have read the section [Add New Translation
Tags](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/HowToAddNewLanguage.md#add-new-translation-tags)
(for new translation tags only)

### UI Changes (if applicable)

- [ ] Screenshots or videos demonstrating the UI changes are attached
(e.g., as comments or direct attachments in the PR)

### Testing (if applicable)

- [ ] I have tested my changes locally. Refer to the [Testing
Guide](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/DeveloperGuide.md#6-testing)
for more details.

src/main/java/stirling/software/SPDF/controller/api/EmailController.java

@@ -0,0 +1,57 @@
+package stirling.software.SPDF.controller.api;
+
+import org.springframework.boot.autoconfigure.condition.ConditionalOnProperty;
+import org.springframework.http.HttpStatus;
+import org.springframework.http.ResponseEntity;
+import org.springframework.web.bind.annotation.ModelAttribute;
+import org.springframework.web.bind.annotation.PostMapping;
+import org.springframework.web.bind.annotation.RequestMapping;
+import org.springframework.web.bind.annotation.RestController;
+
+import io.swagger.v3.oas.annotations.tags.Tag;
+
+import jakarta.mail.MessagingException;
+import jakarta.validation.Valid;
+
+import lombok.RequiredArgsConstructor;
+import lombok.extern.slf4j.Slf4j;
+
+import stirling.software.SPDF.config.security.mail.EmailService;
+import stirling.software.SPDF.model.api.Email;
+
+/**
+ * Controller for handling email-related API requests. This controller exposes an endpoint for
+ * sending emails with attachments.
+ */
+@RestController
+@RequestMapping("/api/v1/general")
+@RequiredArgsConstructor
+@Slf4j
+@Tag(name = "General", description = "General APIs")
+@ConditionalOnProperty(value = "mail.enabled", havingValue = "true", matchIfMissing = false)
+public class EmailController {
+    private final EmailService emailService;
+
+    /**
+     * Endpoint to send an email with an attachment. This method consumes a multipart/form-data
+     * request containing the email details and attachment.
+     *
+     * @param email The Email object containing recipient address, subject, body, and file
+     *     attachment.
+     * @return ResponseEntity with success or error message.
+     */
+    @PostMapping(consumes = "multipart/form-data", value = "/send-email")
+    public ResponseEntity<String> sendEmailWithAttachment(@Valid @ModelAttribute Email email) {
+        try {
+            // Calls the service to send the email with attachment
+            emailService.sendEmailWithAttachment(email);
+            return ResponseEntity.ok("Email sent successfully");
+        } catch (MessagingException e) {
+            // Catches any messaging exception (e.g., invalid email address, SMTP server issues)
+            String errorMsg = "Failed to send email: " + e.getMessage();
+            log.error(errorMsg, e); // Logging the detailed error
+            // Returns an error response with status 500 (Internal Server Error)
+            return ResponseEntity.status(HttpStatus.INTERNAL_SERVER_ERROR).body(errorMsg);
+        }
+    }
+}