implement ota updates

.gitea/workflows/android-release.yml

@@ -74,6 +74,16 @@ jobs:
           printf '%s' "$KEYSTORE_BASE64" | base64 -d > "$RUNNER_TEMP/kadence.keystore"
           echo "signed=true" >> "$GITHUB_OUTPUT"
 
+      - name: Resolve the version name
+        run: |
+          if [[ "$GITHUB_REF" == refs/tags/v* ]]; then
+            VERSION="${GITHUB_REF#refs/tags/v}"
+          else
+            VERSION="0.1.0-master.${{ gitea.run_number }}"
+          fi
+          echo "KADENCE_VERSION_NAME=$VERSION" >> "$GITHUB_ENV"
+          echo "Resolved version $VERSION"
+
       - name: Build the release APK
         working-directory: android
         env:
@@ -103,12 +113,42 @@ jobs:
             fi
           fi
 
+      # A small, separately uploaded description of the APK. The backend's OTA
+      # endpoint reads this artifact on every check; keeping it out of the APK
+      # artifact avoids downloading tens of megabytes just to ask "is there a
+      # newer build?".
+      - name: Write the update metadata
+        run: |
+          APK=android/app/build/outputs/apk/release/app-release.apk
+          META_DIR=android/app/build/outputs/update
+          mkdir -p "$META_DIR"
+
+          APKSIGNER="$(find "$ANDROID_HOME/build-tools" -name apksigner | sort | tail -1)"
+          SIGNER_SHA256="$("$APKSIGNER" verify --print-certs "$APK" \
+            | sed -n 's/^Signer #1 certificate SHA-256 digest: //p' | head -1)"
+
+          printf '{\n  "version_code": %s,\n  "version_name": "%s",\n  "commit": "%s",\n  "built_at": "%s",\n  "size_bytes": %s,\n  "sha256": "%s",\n  "signer_sha256": "%s"\n}\n' \
+            "${{ gitea.run_number }}" \
+            "$KADENCE_VERSION_NAME" \
+            "$GITHUB_SHA" \
+            "$(date -u +%Y-%m-%dT%H:%M:%SZ)" \
+            "$(stat -c%s "$APK")" \
+            "$(sha256sum "$APK" | cut -d' ' -f1)" \
+            "$SIGNER_SHA256" > "$META_DIR/update.json"
+          cat "$META_DIR/update.json"
+
       - uses: actions/upload-artifact@v3
         with:
           name: kadence-release-apk
           path: android/app/build/outputs/apk/release/app-release.apk
           if-no-files-found: error
 
+      - uses: actions/upload-artifact@v3
+        with:
+          name: kadence-update-metadata
+          path: android/app/build/outputs/update/update.json
+          if-no-files-found: error
+
       - name: Attach it to the release, when this is a tag
         if: startsWith(github.ref, 'refs/tags/v')
         env: