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.

build.gradle

@@ -51,16 +51,20 @@ sourceSets {
     main {
         java {
             if (System.getenv("DOCKER_ENABLE_SECURITY") == "false") {
+                exclude "stirling/software/SPDF/config/interfaces/DatabaseInterface.java"
                 exclude "stirling/software/SPDF/config/security/**"
                 exclude "stirling/software/SPDF/controller/api/DatabaseController.java"
-                exclude "stirling/software/SPDF/controller/api/UserController.java"
+                exclude "stirling/software/SPDF/controller/api/EmailController.java"
                 exclude "stirling/software/SPDF/controller/api/H2SQLCondition.java"
+                exclude "stirling/software/SPDF/controller/api/UserController.java"
                 exclude "stirling/software/SPDF/controller/web/AccountWebController.java"
                 exclude "stirling/software/SPDF/controller/web/DatabaseWebController.java"
+                exclude "stirling/software/SPDF/model/api/Email.java"
                 exclude "stirling/software/SPDF/model/ApiKeyAuthenticationToken.java"
                 exclude "stirling/software/SPDF/model/AttemptCounter.java"
                 exclude "stirling/software/SPDF/model/Authority.java"
-                exclude "stirling/software/SPDF/model/BackupNotFoundException.java"
+                exclude "stirling/software/SPDF/model/exception/BackupNotFoundException.java"
+                exclude "stirling/software/SPDF/model/exception/NoProviderFoundException.java"
                 exclude "stirling/software/SPDF/model/PersistentLogin.java"
                 exclude "stirling/software/SPDF/model/SessionEntity.java"
                 exclude "stirling/software/SPDF/model/User.java"
@@ -78,16 +82,8 @@ sourceSets {
         java {
             if (System.getenv("DOCKER_ENABLE_SECURITY") == "false") {
                 exclude "stirling/software/SPDF/config/security/**"
-                exclude "stirling/software/SPDF/controller/api/UserControllerTest.java"
-                exclude "stirling/software/SPDF/controller/api/DatabaseControllerTest.java"
-                exclude "stirling/software/SPDF/controller/web/AccountWebControllerTest.java"
-                exclude "stirling/software/SPDF/controller/web/DatabaseWebControllerTest.java"
                 exclude "stirling/software/SPDF/model/ApiKeyAuthenticationTokenTest.java"
-                exclude "stirling/software/SPDF/model/AttemptCounterTest.java"
-                exclude "stirling/software/SPDF/model/AuthorityTest.java"
-                exclude "stirling/software/SPDF/model/PersistentLoginTest.java"
-                exclude "stirling/software/SPDF/model/SessionEntityTest.java"
-                exclude "stirling/software/SPDF/model/UserTest.java"
+                exclude "stirling/software/SPDF/controller/api/EmailControllerTest.java"
                 exclude "stirling/software/SPDF/repository/**"
             }
 
@@ -459,6 +455,7 @@ dependencies {
         implementation "org.thymeleaf.extras:thymeleaf-extras-springsecurity5:3.1.3.RELEASE"
         implementation "org.springframework.boot:spring-boot-starter-data-jpa:$springBootVersion"
         implementation "org.springframework.boot:spring-boot-starter-oauth2-client:$springBootVersion"
+        implementation "org.springframework.boot:spring-boot-starter-mail:$springBootVersion"
 
         implementation "org.springframework.session:spring-session-core:3.4.3"
         implementation "org.springframework:spring-jdbc:6.2.6"