ci(deps): bump actions/checkout from 4 to 6

Bumps [actions/checkout](https://github.com/actions/checkout) from 4 to 6.
- [Release notes](https://github.com/actions/checkout/releases)
- [Changelog](https://github.com/actions/checkout/blob/main/CHANGELOG.md)
- [Commits](https://github.com/actions/checkout/compare/v4...v6)

---
updated-dependencies:
- dependency-name: actions/checkout
  dependency-version: '6'
  dependency-type: direct:production
  update-type: version-update:semver-major
...

Signed-off-by: dependabot[bot] <support@github.com>

.github/workflows/readiness-gate.yml

@@ -15,7 +15,7 @@ jobs:
     name: Backend Lint & Type Check
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/checkout@v4
+      - uses: actions/checkout@v6
         with:
           submodules: recursive
 
@@ -43,7 +43,7 @@ jobs:
     name: Backend Tests
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/checkout@v4
+      - uses: actions/checkout@v6
         with:
           submodules: recursive
 
@@ -75,7 +75,7 @@ jobs:
     name: Backend Startup Check
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/checkout@v4
+      - uses: actions/checkout@v6
         with:
           submodules: recursive
 
@@ -104,7 +104,7 @@ jobs:
         working-directory: swingmusic_mobile
 
     steps:
-      - uses: actions/checkout@v4
+      - uses: actions/checkout@v6
         with:
           submodules: recursive
 
@@ -131,7 +131,7 @@ jobs:
         working-directory: swingmusic-webclient
 
     steps:
-      - uses: actions/checkout@v4
+      - uses: actions/checkout@v6
         with:
           submodules: recursive
 
@@ -162,7 +162,7 @@ jobs:
         working-directory: swingmusic-desktop
 
     steps:
-      - uses: actions/checkout@v4
+      - uses: actions/checkout@v6
         with:
           submodules: recursive
 
@@ -186,7 +186,7 @@ jobs:
     if: always()
 
     steps:
-      - uses: actions/checkout@v4
+      - uses: actions/checkout@v6
         with:
           submodules: recursive
 

.github/workflows/security.yml

@@ -24,7 +24,7 @@ jobs:
 
     steps:
       - name: Checkout repository
-        uses: actions/checkout@v4
+        uses: actions/checkout@v6
 
       - name: Initialize CodeQL
         uses: github/codeql-action/init@v3
@@ -47,7 +47,7 @@ jobs:
 
     steps:
       - name: Checkout repository
-        uses: actions/checkout@v4
+        uses: actions/checkout@v6
         with:
           submodules: recursive
 
@@ -69,7 +69,7 @@ jobs:
     name: Python Dependency Audit
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/checkout@v4
+      - uses: actions/checkout@v6
 
       - name: Set up Python
         uses: actions/setup-python@v5
@@ -90,7 +90,7 @@ jobs:
       run:
         working-directory: swingmusic-webclient
     steps:
-      - uses: actions/checkout@v4
+      - uses: actions/checkout@v6
         with:
           submodules: recursive
 
@@ -113,7 +113,7 @@ jobs:
       run:
         working-directory: swingmusic-desktop
     steps:
-      - uses: actions/checkout@v4
+      - uses: actions/checkout@v6
         with:
           submodules: recursive
 
@@ -136,7 +136,7 @@ jobs:
     name: Secret Scanning
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/checkout@v4
+      - uses: actions/checkout@v6
         with:
           fetch-depth: 0
 

.github/workflows/unified-release.yml

@@ -31,7 +31,7 @@ jobs:
       release_notes: ${{ steps.version.outputs.release_notes }}
     
     steps:
-    - uses: actions/checkout@v4
+    - uses: actions/checkout@v6
       with:
         fetch-depth: 0
     
@@ -164,7 +164,7 @@ jobs:
             rust_target: 'x86_64-unknown-linux-gnu'
     
     steps:
-    - uses: actions/checkout@v4
+    - uses: actions/checkout@v6
     - name: Initialize submodules
       run: git submodule update --init --recursive
     
@@ -231,7 +231,7 @@ jobs:
             rust_target: 'x86_64-pc-windows-gnu'
     
     steps:
-    - uses: actions/checkout@v4
+    - uses: actions/checkout@v6
     - name: Initialize submodules
       run: git submodule update --init --recursive
     
@@ -300,7 +300,7 @@ jobs:
             rust_target: 'aarch64-apple-darwin'
     
     steps:
-    - uses: actions/checkout@v4
+    - uses: actions/checkout@v6
     - name: Initialize submodules
       run: git submodule update --init --recursive
     
@@ -355,7 +355,7 @@ jobs:
     if: contains(github.event.inputs.components, 'mobile') || github.event_name == 'push'
     
     steps:
-    - uses: actions/checkout@v4
+    - uses: actions/checkout@v6
     - name: Initialize submodules
       run: git submodule update --init --recursive
     
@@ -392,7 +392,7 @@ jobs:
     if: contains(github.event.inputs.components, 'backend') || github.event_name == 'push'
     
     steps:
-    - uses: actions/checkout@v4
+    - uses: actions/checkout@v6
     - name: Initialize submodules
       run: git submodule update --init --recursive
     
@@ -439,7 +439,7 @@ jobs:
     if: success()
     
     steps:
-    - uses: actions/checkout@v4
+    - uses: actions/checkout@v6
     
     - name: Download all artifacts
       uses: actions/download-artifact@v4