ci: migrate workflows from GitHub Actions to Gitea Actions

- Move CI workflow from .github/workflows/ to .gitea/workflows/
- Add Gitea Actions release workflow for tag builds
- Remove GitHub-specific release workflow

.gitea/workflows/release.yml

@@ -0,0 +1,20 @@
+name: Release
+
+on:
+  push:
+    tags:
+      - "v*"
+
+jobs:
+  build:
+    runs-on: ubuntu-latest
+
+    steps:
+      - name: Checkout repository
+        uses: actions/checkout@v4
+
+      - name: Build Docker image
+        run: docker build ./backend --tag aoc-backend:${{ gitea.ref_name }}
+
+      - name: Verify image exists
+        run: docker images | grep aoc-backend