ci: sync release workflow with CI/CD docs, add manual build

- Fix github.* → gitea.* context consistency
- Add pre-release detection (skip :latest for alpha/beta/rc)
- Add release fallback (reuse existing if creation fails)
- Add prerelease field to release API call
- Use sparse-checkout for RELEASE_NOTES.md
- Skip Portainer redeploy for pre-releases
- Add version tag without v prefix
- Add manual build.yml for Docker image verification

.gitea/workflows/build.yml

@@ -0,0 +1,13 @@
+name: Build Docker Image
+
+on:
+  workflow_dispatch:
+
+jobs:
+  build:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+
+      - name: Build Docker image
+        run: docker build -t notify-bridge:dev .