Add flake update workflow

.gitea/workflows/flake-update.yml

@@ -0,0 +1,45 @@
+name: Update nix flake inputs
+
+on:
+  schedule:
+    # Every Sunday at 03:00 UTC
+    - cron: "0 3 * * 0"
+
+jobs:
+  flake-update:
+    runs-on: ubuntu-latest
+
+    permissions:
+      contents: write
+
+    steps:
+      - name: Git Config
+        run: |
+          git config user.name "gitea-actions"
+          git config user.email "actions@${GITEA_SERVER_HOST:-gitea.local}"
+
+      - name: Checkout repository
+        uses: actions/checkout@v4
+        with:
+          fetch-depth: 0
+
+      - name: Install Nix
+        uses: DeterminateSystems/nix-installer-action@v12
+        with:
+          nix_path: nixpkgs=channel:nixos-unstable
+          extra_conf: |
+            experimental-features = nix-command flakes
+
+      - name: Update flake.lock
+        run: nix flake update
+
+      - name: Commit flake.lock
+        run: |
+          if git diff --quiet flake.lock; then
+            echo "No changes to flake.lock"
+            exit 0
+          fi
+
+          git add flake.lock
+          git commit -m "Update flake inputs"
+          git push