ci: add OTA build workflow

.gitea/workflows/ota-build.yml

@@ -0,0 +1,46 @@
+name: OTA build
+
+on:
+  push:
+    branches:
+      - '**'
+  workflow_dispatch:
+
+jobs:
+  build-ota:
+    runs-on: ubuntu-latest
+    timeout-minutes: 45
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v4
+
+      - name: Set up Python
+        uses: actions/setup-python@v5
+        with:
+          python-version: '3.11'
+
+      - name: Install PlatformIO
+        run: |
+          python -m pip install --upgrade pip
+          pip install platformio
+
+      - name: Build OTA firmware binaries
+        run: pio run
+
+      - name: Collect OTA artifacts
+        run: |
+          mkdir -p dist/ota
+          find .pio/build -type f -name 'firmware.bin' -exec cp {} dist/ota/ \;
+          count=$(find dist/ota -type f -name '*.bin' | wc -l)
+          if [ "$count" -eq 0 ]; then
+            echo 'No firmware.bin artifacts were produced.' >&2
+            exit 1
+          fi
+          find dist/ota -maxdepth 1 -type f | sort
+
+      - name: Upload OTA artifacts
+        uses: actions/upload-artifact@v3
+        with:
+          name: ota-binaries
+          path: dist/ota/*.bin
+          if-no-files-found: error