refactor: move modules under app/ directory and update file paths (#3938)

# Description of Changes

- **What was changed:**  
- Renamed top-level directories: `stirling-pdf` → `app/core`, `common` →
`app/common`, `proprietary` → `app/proprietary`.
- Updated all path references in `.gitattributes`, GitHub workflows
(`.github/workflows/*`), scripts (`.github/scripts/*`), `.gitignore`,
Dockerfiles, license files, and template settings to reflect the new
structure.
- Added a new CI job `check-generateOpenApiDocs` to generate and upload
OpenAPI documentation.
- Removed redundant `@Autowired` annotations from `TempFileShutdownHook`
and `UnlockPDFFormsController`.
- Minor formatting and comment adjustments in YAML templates and
resource files.

- **Why the change was made:**  
- To introduce a clear `app/` directory hierarchy for core, common, and
proprietary modules, improving organization and maintainability.
- To ensure continuous integration and Docker builds continue to work
seamlessly with the reorganized structure.
- To automate OpenAPI documentation generation as part of the CI
pipeline.

---

## 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.

.github/workflows/sync_files.yml

@@ -8,8 +8,8 @@ on:
     paths:
       - "build.gradle"
       - "README.md"
-      - "stirling-pdf/src/main/resources/messages_*.properties"
-      - "stirling-pdf/src/main/resources/static/3rdPartyLicenses.json"
+      - "app/core/src/main/resources/messages_*.properties"
+      - "app/core/src/main/resources/static/3rdPartyLicenses.json"
       - "scripts/ignore_translation.toml"
 
 permissions:
@@ -41,11 +41,11 @@ jobs:
 
       - name: Sync translation property files
         run: |
-          python .github/scripts/check_language_properties.py --reference-file "stirling-pdf/src/main/resources/messages_en_GB.properties" --branch main
+          python .github/scripts/check_language_properties.py --reference-file "app/core/src/main/resources/messages_en_GB.properties" --branch main
 
       - name: Commit translation files
         run: |
-          git add stirling-pdf/src/main/resources/messages_*.properties
+          git add app/core/src/main/resources/messages_*.properties
           git diff --staged --quiet || git commit -m ":memo: Sync translation files" || echo "No changes detected"
 
       - name: Install dependencies
@@ -101,4 +101,4 @@ jobs:
           sign-commits: true
           add-paths: |
             README.md
-              stirling-pdf/src/main/resources/messages_*.properties
+            app/core/src/main/resources/messages_*.properties