ci: Consolidate workflows and optimise build pipeline performance

Merge rust-checks.yml and release-image.yml into unified ci-build.yml
workflow that runs faster and more efficiently. The previous setup ran
4+ parallel jobs immediately (format, clippy, test, builds), causing
resource contention. The new pipeline runs max 2 jobs in parallel at
each stage, catching lint/format issues quickly before attempting
expensive compilation.

Extract all Rust setup logic from both workflows into reusable
rust-with-cache composite action. This replaces 6 separate actions
(rust-toolchain, sccache, timelord, plus inline APT/cache steps) with
a single action that handles:
- Rust toolchain installation with component selection
- Cross-compilation configuration (previously scattered across
  release-image.yml)
- System dependency installation with proper error handling
- Comprehensive caching (sccache, cargo registry, cargo target, uv
  tools)
- Timeline tracking and performance monitoring

The previous release-image.yml had cross-compilation support but it
was implemented inline with complex environment variables. The new
rust-with-cache action centralises this with proper parameters for
pkg-config paths, foreign architecture setup, and toolchain selection.

Performance improvements make the pipeline fast enough to consolidate:
- Warmed sccache cache shared between check and build stages
- Optimised cargo target cache to exclude incremental/ and binaries
  (was caching entire target/ directory via buildkit-cache-dance)
- Add restore-keys fallback for better cache hit rates
- Parallel background tasks for Rust setup while APT runs
- Fail-fast on format/lint errors before expensive compilation

Fix ARM64 cross-compilation reliability issues:
- Move APT installations from background to foreground (background
  processes would hang during package downloads despite
  DEBIAN_FRONTEND=noninteractive)
- Set proper pkg-config environment for cross-compilation
- Configure APT sources to ports.ubuntu.com for foreign architectures
- Replace hardened_malloc with jemalloc (ARM64 unsupported)

Modernisation from previous commit (b0ebdb59):
- prefligit renamed to prek (avoid typosquatting)
- Direct uvx rustup replacing custom rust-toolchain action
- Workflow renames: deploy-element, deploy-docs, docker-mirror
- Renovate configuration for .forgejo/ workflows
- fix-byte-order-marker replacing check-byte-order-marker

Docker improvements:
- Remove buildkit-cache-dance injection (now handled natively)
- Align tag naming between arch-specific and multi-platform builds
- Add branch- prefix for non-default branches
- Reserve latest-{arch} tags for version releases only

The consolidated pipeline completes in ~10 minutes with better
resource utilisation and clearer failure diagnostics. Both x86_64 and
ARM64 builds now work reliably with the centralised cross-compilation
configuration.

.forgejo/workflows/renovate.yml

@@ -0,0 +1,60 @@
+name: Maintenance / Renovate
+on:
+  schedule:
+    # Run at 2am UTC daily
+    - cron: '0 2 * * *'
+  workflow_dispatch:
+    inputs:
+      dryRun:
+        description: 'Dry run mode'
+        required: false
+        default: 'false'
+        type: choice
+        options:
+          - 'true'
+          - 'false'
+      logLevel:
+        description: 'Log level'
+        required: false
+        default: 'info'
+        type: choice
+        options:
+          - 'debug'
+          - 'info'
+          - 'warn'
+          - 'error'
+  push:
+    branches:
+      - main
+    paths:
+      - '.forgejo/workflows/renovate.yml'
+      - 'renovate.json'
+
+jobs:
+  renovate:
+    name: Renovate
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v4
+
+      - name: Run Renovate
+        uses: renovatebot/github-action@v40.1.0
+        with:
+          token: ${{ secrets.RENOVATE_TOKEN }}
+          configurationFile: renovate.json
+        env:
+          # Platform settings
+          RENOVATE_PLATFORM: gitea
+          RENOVATE_ENDPOINT: ${{ github.server_url }}/api/v1
+          RENOVATE_TOKEN: ${{ secrets.RENOVATE_TOKEN }}
+
+          # Repository settings
+          RENOVATE_REPOSITORIES: '["${{ github.repository }}"]'
+
+          # Behaviour settings
+          RENOVATE_DRY_RUN: ${{ inputs.dryRun || 'false' }}
+          LOG_LEVEL: ${{ inputs.logLevel || 'info' }}
+
+          # Forgejo/Gitea specific
+          RENOVATE_GIT_AUTHOR: '${{ vars.RENOVATE_AUTHOR }}'