ci: pin GitHub Actions to commit SHAs and add Dependabot

Pin every action reference to the full commit SHA that its current tag
already resolves to, with the version kept in a trailing comment. No action
changes version as a result of this commit -- including actions/setup-node
in validate_issue.yml, which stays on v4.4.0 rather than being lifted to the
v6 used by the other workflows. Version bumps are left to Dependabot so they
arrive as reviewable PRs.

Add .github/dependabot.yml for the github-actions ecosystem: weekly on
Monday, minor/patch grouped into a single PR (majors always get their own),
"ci" commit prefix. tibdex/github-app-token is ignored to match
iptv-org-database; it is archived upstream, so replacing it should be a
considered migration rather than an automated bump.

.github/workflows/validate_label.yml

@@ -12,9 +12,9 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - name: Checkout repository code
-        uses: actions/checkout@v6
+        uses: actions/checkout@d23441a48e516b6c34aea4fa41551a30e30af803 # v6.1.0
 
-      - uses: tibdex/github-app-token@v1.8.2
+      - uses: tibdex/github-app-token@0d49dd721133f900ebd5e0dff2810704e8defbc6 # v1.8.2
         if: ${{ !env.ACT }}
         id: create-app-token
         with: