chore: add weekly flake.lock update workflow

Adds GitHub Action to automatically update flake.lock every Sunday.
Uses DeterminateSystems/update-flake-lock to create PRs with updated
dependencies, preventing large painful updates like the recent 2-year gap.

Targets develop branch with automated and dependencies labels.
Can also be triggered manually via workflow_dispatch.

Author: yvan-sraka

.github/workflows/update-flake-lock.yml

@@ -0,0 +1,33 @@
+name: Update Flake Lock
+
+on:
+  workflow_dispatch:
+  schedule:
+    - cron: '0 0 * * 0'
+
+jobs:
+  update-flake-lock:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout repository
+        uses: actions/checkout@v4
+
+      - name: Install Nix
+        uses: DeterminateSystems/nix-installer-action@v14
+
+      - name: Update flake.lock
+        uses: DeterminateSystems/update-flake-lock@v24
+        with:
+          pr-title: "chore: update flake.lock"
+          pr-body: |
+            Automated update of flake.lock dependencies.
+
+            This PR updates all flake inputs to their latest versions.
+
+            Please review the changes and ensure CI passes before merging.
+          pr-labels: |
+            dependencies
+            automated
+          branch: update-flake-lock
+          base-branch: develop
+          token: ${{ secrets.GITHUB_TOKEN }}