ci: Gitea Actions build + lftp SFTP deploy

Triggers on push to main and manual workflow_dispatch. Builds with
Node 20, runs astro check, builds the static site, then mirrors
./dist via SFTP using the SFTP_* secrets configured on the repo.

Password-based SFTP for now (matches the credentials we have). Swap to
key-based by adding SFTP_KEY as a secret and tweaking the lftp call
when the production site user is created.

.gitea/workflows/deploy.yml

@@ -0,0 +1,63 @@
+name: Build and deploy
+
+on:
+  push:
+    branches: [main]
+  workflow_dispatch:
+
+jobs:
+  deploy:
+    runs-on: ubuntu-latest
+    timeout-minutes: 10
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v4
+
+      - name: Setup Node
+        uses: actions/setup-node@v4
+        with:
+          node-version: 20
+          cache: npm
+
+      - name: Install dependencies
+        run: npm ci
+
+      - name: Type and link check
+        run: npm run check
+
+      - name: Build
+        run: npm run build
+
+      - name: Install lftp
+        run: |
+          sudo apt-get update -qq
+          sudo apt-get install -y --no-install-recommends lftp
+
+      - name: SFTP mirror via lftp
+        env:
+          SFTP_HOST: ${{ secrets.SFTP_HOST }}
+          SFTP_PORT: ${{ secrets.SFTP_PORT }}
+          SFTP_USER: ${{ secrets.SFTP_USER }}
+          SFTP_PASS: ${{ secrets.SFTP_PASS }}
+          SFTP_PATH: ${{ secrets.SFTP_PATH }}
+        run: |
+          lftp -e "
+            set sftp:auto-confirm yes;
+            set ssl:verify-certificate no;
+            open -u '$SFTP_USER,$SFTP_PASS' sftp://$SFTP_HOST:$SFTP_PORT;
+            mirror --reverse --delete --parallel=4 --verbose \
+                   --exclude-glob '.well-known/' \
+                   ./dist/ $SFTP_PATH;
+            bye
+          "
+
+      - name: Summary
+        if: always()
+        run: |
+          {
+            echo "### Deployed"
+            echo ""
+            echo "- Commit: ${{ github.sha }}"
+            echo "- Page count: $(find dist -name '*.html' 2>/dev/null | wc -l)"
+            echo "- Build size: $(du -sh dist 2>/dev/null | cut -f1)"
+          } >> "$GITHUB_STEP_SUMMARY"