ci: Tidy CI pipeline for readability and performance

Pipeline Modernisation:
- Update prefligit to prek (same project, renamed to avoid
  typosquatting)
- Replace custom rust-toolchain action with direct uvx rustup
  invocation
- Remove dependency on install scripts in favour of uvx tool
  execution
- Make sccache conditional on GH_APP_ID and GH_APP_PRIVATE_KEY
  availability

Workflow Restructuring:
- Rename workflows for improved clarity:
  * rust-checks.yml → ci-checks.yml (consolidates Rust + prek)
  * element.yml → deploy-element.yml
  * documentation.yml → deploy-docs.yml
  * release-image.yml → release-builds.yml (Docker + binaries)
  * mirror-images.yml → docker-mirror.yml
- Delete obsolete prefligit-checks.yml (merged into ci-checks.yml)
- Combine format and prek checks into single fast-checks job

Cache Strategy Improvements (release-builds.yml):
- Add restore-keys to Rust registry and cargo target caches
- Include Cargo.toml files in cargo-target cache key
- Consolidate separate apt-cache and apt-lib steps into single step
- Version APT cache with Dockerfile hash instead of static keys
- Add platform-specific Docker buildcache tags
- Simplify Rust registry paths (remove checkouts/src subdirs)

CI Execution Improvements:
- Add uv cache for faster uvx tool invocations (prek, rustup)
- Enable concurrency control with cancel-in-progress
- Fix BOM handling in pre-commit

Dependency Management:
- Add renovate.yml workflow for scheduled dependency checking
- Configure renovate.json to monitor .forgejo/ and .github/ dirs
- Group non-major GitHub Actions updates into single PRs
- Set PR limits: 3 concurrent, 2 per hour

.forgejo/workflows/docker-mirror.yml

@@ -0,0 +1,47 @@
+name: Deploy / Mirror Images
+
+on:
+  schedule:
+    # Run every 2 hours
+    - cron: "0 */2 * * *"
+  workflow_dispatch:
+    inputs:
+      dry_run:
+        description: 'Dry run (check only, no actual mirroring)'
+        required: false
+        default: false
+        type: boolean
+
+concurrency:
+  group: "mirror-images"
+  cancel-in-progress: true
+
+jobs:
+  mirror-images:
+    runs-on: ubuntu-latest
+    env:
+      BUILTIN_REGISTRY_USER: ${{ vars.BUILTIN_REGISTRY_USER }}
+      BUILTIN_REGISTRY_PASSWORD: ${{ secrets.BUILTIN_REGISTRY_PASSWORD }}
+      GITLAB_USERNAME: ${{ vars.GITLAB_USERNAME }}
+      GITLAB_TOKEN: ${{ secrets.GITLAB_TOKEN }}
+    steps:
+      - name: Checkout repository
+        uses: actions/checkout@v4
+        with:
+          persist-credentials: false
+
+      - name: Install regctl
+        uses: https://forgejo.ellis.link/continuwuation/regclient-actions/regctl-installer@main
+        with:
+          binary: regsync
+
+      - name: Check what images need mirroring
+        run: |
+          echo "Checking images that need mirroring..."
+          regsync check -c .forgejo/regsync/regsync.yml -v info
+
+      - name: Mirror images
+        if: ${{ !inputs.dry_run }}
+        run: |
+          echo "Starting image mirroring..."
+          regsync once -c .forgejo/regsync/regsync.yml -v info