ci: Modernise CI pipeline with improved tooling and workflow structure

Pipeline Modernisation:
- Updated to prek (prefligit renamed to avoid typosquatting)
- Adopted uvx for tool invocation (prek, rustup) without install scripts
- Consolidated workflow steps to run in parallel where possible
- Removed redundant custom actions in favour of direct uvx commands
- Made sccache optional (based on secrets availability) for fork compatibility

Workflow Restructuring:
- Renamed workflows for clarity and consistency:
  * rust-checks.yml → ci-checks.yml (now includes prek linting)
  * clients-element.yml → deploy-element.yml
  * documentation.yml → deploy-docs.yml
  * release-image.yml → release-builds.yml (builds images AND binaries)
  * mirror-images.yml → docker-mirror.yml
- Simplified job and step naming throughout for better readability

Performance Improvements:
- Parallel execution of independent steps (formatting and prek checks)
- Optimised caching strategy for uv and Rust dependencies
- Reduced CI execution time through better parallelisation

Dependency Management:
- Added Renovate configuration for automated dependency updates
- Configured to monitor both .forgejo/ and .github/ workflows
- Set up automatic PR creation for non-major GitHub Actions updates

.forgejo/workflows/mirror-images.yml

@@ -1,47 +0,0 @@
-name: Mirror Container 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