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/)

.github/workflows/release-please.yml

@@ -1,52 +1,13 @@
-name: Release Please
+# Deprecated: replaced by .gitea/workflows/docker-build.yml (tag-trigger).
+# Origin moved from GitHub to Gitea; release-please-action and ghcr.io are
+# GitHub-only and don't run on Gitea Actions. Trigger changed to manual-only
+# so neither GitHub nor Gitea Actions will auto-run this file.
+name: Release Please (deprecated)
 on:
-  push:
-    branches: [main]
-
-permissions:
-  contents: write
-  pull-requests: write
+  workflow_dispatch:
 
 jobs:
-  release-please:
+  noop:
     runs-on: ubuntu-latest
-    outputs:
-      release_created: ${{ steps.release.outputs.release_created }}
-      tag_name: ${{ steps.release.outputs.tag_name }}
     steps:
-      - uses: googleapis/release-please-action@v4
-        id: release
-        with:
-          release-type: node
-
-  docker:
-    needs: release-please
-    if: ${{ needs.release-please.outputs.release_created }}
-    runs-on: ubuntu-latest
-    permissions:
-      contents: read
-      packages: write
-    steps:
-      - uses: actions/checkout@v4
-
-      - uses: docker/login-action@v3
-        with:
-          registry: ghcr.io
-          username: ${{ github.actor }}
-          password: ${{ secrets.GITHUB_TOKEN }}
-
-      - uses: docker/metadata-action@v5
-        id: meta
-        with:
-          images: ghcr.io/${{ github.repository }}
-          tags: |
-            type=semver,pattern={{version}},value=${{ needs.release-please.outputs.tag_name }}
-            type=semver,pattern={{major}}.{{minor}},value=${{ needs.release-please.outputs.tag_name }}
-            type=raw,value=latest
-
-      - uses: docker/build-push-action@v6
-        with:
-          context: .
-          push: true
-          tags: ${{ steps.meta.outputs.tags }}
-          labels: ${{ steps.meta.outputs.labels }}
+      - run: echo "Deprecated. See .gitea/workflows/docker-build.yml"