chore(ci): optimize GitHub Actions Gradle caching across workflows (#7299)

# Description of Changes

This PR refactors Gradle caching across the GitHub Actions workflows to
improve cache reuse, reduce dependency resolution overhead, and shorten
CI execution times.

### What was changed

- Replaced multiple `gradle/actions/setup-gradle` steps with a unified
`actions/cache`-based Gradle User Home cache strategy.
- Standardized cache paths across workflows to include:
  - `~/.gradle/caches`
  - `~/.gradle/wrapper`
- Introduced consistent cache keys using:
  - Runner OS
  - Runner architecture
  - JDK version
- Hashes of Gradle wrapper, version catalog, Gradle build files, and
project build scripts.
- Added restore keys to maximize cache hit rates across similar
environments.
- Added a new **`gradle-cache-prime`** job in the main build workflow
that:
  - Restores or creates the shared Gradle cache.
  - Resolves backend dependencies before downstream jobs execute.
  - Makes the populated cache available to subsequent jobs.
- Updated workflow dependencies so Gradle-based jobs wait for the cache
priming job before execution.
- Simplified and unified Gradle cache handling across numerous CI
workflows, including backend builds, OpenAPI generation, database
migration tests, Docker tests, Tauri builds, Swagger generation,
enterprise builds, release workflows, and license generation.
- Updated workflow comments to reflect the new caching strategy and
shared cache behavior.

### Why the change was made

The previous workflows used a mixture of Gradle setup actions and
partial dependency caches, leading to duplicated dependency downloads,
inconsistent cache behavior, and longer CI runtimes. Consolidating all
workflows onto a shared Gradle User Home cache with a dedicated cache
priming job improves cache reuse, reduces unnecessary dependency
resolution, and makes CI execution more consistent.



---

## Checklist

### General

- [ ] I have read the [Contribution
Guidelines](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/CONTRIBUTING.md)
- [ ] 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/devGuide/HowToAddNewLanguage.md)
(if applicable)
- [ ] I have performed a self-review of my own code
- [ ] 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)

### Translations (if applicable)

- [ ] I ran
[`scripts/counter_translation.py`](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/docs/counter_translation.md)

### 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 run `task check` to verify linters, typechecks, and tests
pass
- [ ] I have tested my changes locally. Refer to the [Testing
Guide](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/DeveloperGuide.md#7-testing)
for more details.

.github/workflows/coverage-aggregate.yml

@@ -46,20 +46,16 @@ jobs:
           java-version: "25"
           distribution: "temurin"
 
-      - name: Cache Gradle dependency artifacts
+      - name: Cache Gradle User Home
         uses: actions/cache@55cc8345863c7cc4c66a329aec7e433d2d1c52a9 # v6.1.0
         with:
           path: |
+            ~/.gradle/caches
             ~/.gradle/wrapper
-            ~/.gradle/caches/modules-2/files-2.1
-            ~/.gradle/caches/modules-2/metadata-2.*
-          key: gradle-deps-${{ runner.os }}-jdk-25-${{ hashFiles('**/gradle/wrapper/gradle-wrapper.properties', '**/*.gradle', '**/*.gradle.kts', 'settings.gradle', 'settings.gradle.kts', 'gradle/libs.versions.toml') }}
-
-      - name: Setup Gradle
-        uses: gradle/actions/setup-gradle@3f131e8634966bd73d06cc69884922b02e6faf92 # v6.2.0
-        with:
-          gradle-version: 9.6.1
-          cache-disabled: true
+          key: gradle-${{ runner.os }}-${{ runner.arch }}-jdk-25-${{ hashFiles('gradle/wrapper/gradle-wrapper.properties', 'gradle/libs.versions.toml', 'settings.gradle', 'build.gradle', 'app/**/build.gradle', 'gradle/**/*.gradle') }}
+          restore-keys: |
+            gradle-${{ runner.os }}-${{ runner.arch }}-jdk-25-
+            gradle-${{ runner.os }}-${{ runner.arch }}-
 
       - name: Set up Python
         uses: actions/setup-python@5fda3b95a4ea91299a34e894583c3862153e4b97 # v7.0.0
@@ -81,7 +77,7 @@ jobs:
       # Each lands as a sibling dir under coverage-execs/, with the .exec
       # files preserving their original relative paths.
       - name: Download all .exec artifacts
-        uses: actions/download-artifact@d3f86a106a0bac45b974a628896c90dbdf5c8093 # v6.0.0
+        uses: actions/download-artifact@3e5f45b2cfb9172054b4087a40e8e0b5a5461e7c # v8.0.1
         with:
           pattern: jacoco-exec-*
           path: coverage-execs/
@@ -206,7 +202,7 @@ jobs:
         # absence on backend-only runs by skipping the download entirely
         # when the producer job was not part of this workflow run.
         if: inputs.frontend-validation-result == 'success'
-        uses: actions/download-artifact@d3f86a106a0bac45b974a628896c90dbdf5c8093 # v6.0.0
+        uses: actions/download-artifact@3e5f45b2cfb9172054b4087a40e8e0b5a5461e7c # v8.0.1
         with:
           name: frontend-coverage
           path: matrix-inputs/vitest/
@@ -216,7 +212,7 @@ jobs:
         # e2e-live uploads the artifact with a stable name. Skip the
         # download entirely when the producer job did not run.
         if: inputs.playwright-e2e-live-result == 'success'
-        uses: actions/download-artifact@d3f86a106a0bac45b974a628896c90dbdf5c8093 # v6.0.0
+        uses: actions/download-artifact@3e5f45b2cfb9172054b4087a40e8e0b5a5461e7c # v8.0.1
         with:
           name: playwright-frontend-coverage
           path: matrix-inputs/playwright/