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/model/api/Email.java

@@ -0,0 +1,39 @@
+package stirling.software.SPDF.model.api;
+
+import org.springframework.boot.autoconfigure.condition.ConditionalOnProperty;
+
+import io.swagger.v3.oas.annotations.media.Schema;
+
+
+import lombok.Data;
+import lombok.EqualsAndHashCode;
+import lombok.NoArgsConstructor;
+
+@Data
+@NoArgsConstructor
+@EqualsAndHashCode(callSuper = true)
+@ConditionalOnProperty(value = "mail.enabled", havingValue = "true", matchIfMissing = false)
+public class Email extends GeneralFile {
+
+    @Schema(
+            description = "The recipient's email address",
+            requiredMode = Schema.RequiredMode.REQUIRED,
+            format = "email")
+    private String to;
+
+    @Schema(
+            description = "The subject of the email",
+            defaultValue = "Stirling Software PDF Notification",
+            requiredMode = Schema.RequiredMode.NOT_REQUIRED)
+    private String subject;
+
+    @Schema(
+            description = "The body of the email",
+            requiredMode = Schema.RequiredMode.NOT_REQUIRED,
+            defaultValue =
+                    "This message was automatically generated by Stirling-PDF, an innovative"
+                            + " solution from Stirling Software. For more information, visit our <a"
+                            + " href=\"https://stirling-software.com\">website</a>.<br><br>Please do"
+                            + " not reply directly to this email.")
+    private String body;
+}