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

renovate.json

@@ -22,5 +22,24 @@
     "tikv-jemalloc-ctl",
     "opentelemetry-rust",
     "tracing-opentelemetry"
-  ]
+  ],
+  "github-actions": {
+    "enabled": true,
+    "fileMatch": [
+      "(^|/)\\.forgejo/workflows/[^/]+\\.ya?ml$",
+      "(^|/)\\.forgejo/actions/[^/]+/action\\.ya?ml$",
+      "(^|/)\\.github/workflows/[^/]+\\.ya?ml$",
+      "(^|/)\\.github/actions/[^/]+/action\\.ya?ml$"
+    ]
+  },
+  "packageRules": [
+    {
+      "description": "Group all non-major GitHub Actions updates",
+      "matchManagers": ["github-actions"],
+      "matchUpdateTypes": ["minor", "patch"],
+      "groupName": "github-actions-non-major"
+    }
+  ],
+  "prConcurrentLimit": 3,
+  "prHourlyLimit": 2
 }