feat(common): add configurable maxDPI limit for PDF-to-image conversion (#4129)

# Description of Changes

- **What was changed:**  
Added a new `maxDPI` property under `system` in `ApplicationProperties`;
updated `PdfUtils` to retrieve and enforce this configurable limit
instead of a hard-coded constant; modified `ConverterWebController` and
the PDF-to-image template to expose the limit to users; added
`pdfToImage.dpi` entries across all translation files; updated
`settings.yml.template` and `DeveloperGuide.md` to document the new
setting.
- **Why the change was made:**  
To allow deployments to tune the maximum DPI for PDF-to-image
conversions based on available resources, preventing excessive memory
usage and crashes caused by arbitrarily high DPI values.

Closes #3985

---

## 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/devGuide/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/devGuide/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/devGuide/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/devGuide/DeveloperGuide.md#6-testing)
for more details.

---------

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

app/common/src/main/java/stirling/software/common/model/ApplicationProperties.java

@@ -311,6 +311,7 @@ public class ApplicationProperties {
         private Boolean enableAnalytics;
         private Datasource datasource;
         private Boolean disableSanitize;
+        private int maxDPI;
         private Boolean enableUrlToPDF;
         private Html html = new Html();
         private CustomPaths customPaths = new CustomPaths();

app/common/src/main/java/stirling/software/common/util/PdfUtils.java

@@ -35,6 +35,7 @@ import io.github.pixee.security.Filenames;
 
 import lombok.extern.slf4j.Slf4j;
 
+import stirling.software.common.model.ApplicationProperties;
 import stirling.software.common.service.CustomPDFDocumentFactory;
 
 @Slf4j
@@ -145,13 +146,18 @@ public class PdfUtils {
             throws IOException, Exception {
 
         // Validate and limit DPI to prevent excessive memory usage
-        final int MAX_SAFE_DPI = 500; // Maximum safe DPI to prevent memory issues
-        if (DPI > MAX_SAFE_DPI) {
+        int maxSafeDpi = 500; // Default maximum safe DPI
+        ApplicationProperties properties =
+                ApplicationContextProvider.getBean(ApplicationProperties.class);
+        if (properties != null && properties.getSystem() != null) {
+            maxSafeDpi = properties.getSystem().getMaxDPI();
+        }
+        if (DPI > maxSafeDpi) {
             throw ExceptionUtils.createIllegalArgumentException(
                     "error.dpiExceedsLimit",
                     "DPI value {0} exceeds maximum safe limit of {1}. High DPI values can cause memory issues and crashes. Please use a lower DPI value.",
                     DPI,
-                    MAX_SAFE_DPI);
+                    maxSafeDpi);
         }
 
         try (PDDocument document = pdfDocumentFactory.load(inputStream)) {