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

.pre-commit-config.yaml

@@ -9,7 +9,7 @@ repos:
   - repo: https://github.com/pre-commit/pre-commit-hooks
     rev: v5.0.0
     hooks:
-        - id: check-byte-order-marker
+        - id: fix-byte-order-marker
         - id: check-case-conflict
         - id: check-symlinks
         - id: destroyed-symlinks