ci: migrate to Gitea Actions for docker build

Origin moved from GitHub to Gitea. release-please-action and ghcr.io are
GitHub-specific and don't work on Gitea Actions, so the release-please
flow is dropped in favor of manual tag-trigger docker builds.

- Add .gitea/workflows/docker-build.yml: builds and pushes to
  gitea.shcizo.se/shcizo/pdga-rating on tag push (v*)
- Remove release-please-config.json and .release-please-manifest.json
- Neutralize .github/workflows/release-please.yml to workflow_dispatch
  (kept as reference; can't be deleted via tooling under .github/)

.gitea/workflows/docker-build.yml

@@ -0,0 +1,37 @@
+name: Docker Build
+
+on:
+  push:
+    tags:
+      - 'v*'
+
+jobs:
+  docker:
+    runs-on: ubuntu-latest
+    permissions:
+      contents: read
+      packages: write
+    steps:
+      - uses: actions/checkout@v4
+
+      - uses: docker/login-action@v3
+        with:
+          registry: gitea.shcizo.se
+          username: ${{ gitea.actor }}
+          password: ${{ secrets.GITHUB_TOKEN }}
+
+      - uses: docker/metadata-action@v5
+        id: meta
+        with:
+          images: gitea.shcizo.se/shcizo/pdga-rating
+          tags: |
+            type=semver,pattern={{version}}
+            type=semver,pattern={{major}}.{{minor}}
+            type=raw,value=latest
+
+      - uses: docker/build-push-action@v6
+        with:
+          context: .
+          push: true
+          tags: ${{ steps.meta.outputs.tags }}
+          labels: ${{ steps.meta.outputs.labels }}