Move to Gitea: git.ryuvia.com/niklas/terdut-server

The module path, the container image, the Helm chart and the CI pipeline all
named GitHub. They now name the Gitea instance everything else already runs on.

The workflows are rewritten rather than translated. Gitea's runner image is
ubuntu:22.04, whose nodejs is Node 12, so no JS action runs there at all --
actions/checkout@v4 dies with a SyntaxError before it does anything. Every step
is shell, checkout is a plain clone (this repo is public, so it needs no
credential), and the jobs that need docker or helm run in host mode because the
dind bridge a `container:` job gets cannot reach github.com or get.helm.sh.

Two consequences worth naming:

- upload-artifact/download-artifact are also JS actions, and there is no
  artifact store here, so the job that builds the binaries is the job that
  publishes them. Nothing is passed between jobs.
- setup-qemu-action is gone with the rest, and the runner has no binfmt
  registration. The Dockerfile's builder stage now runs on $BUILDPLATFORM and
  cross-compiles from TARGETARCH instead, which is what keeps the arm64 image
  buildable -- and makes it native rather than emulated.

The chart moves from a GitHub Pages index to an OCI artifact in Gitea's
registry. Publishing stays tag-only for the reason recorded in release.yaml: a
workflow triggered by the branch push cannot know the version it is about to be
tagged with.

The GitHub repository is left in place and untouched. Nothing pushes to it any
more, but its existing release downloads and chart index keep resolving.

.gitea/workflows/ci.yaml

@@ -0,0 +1,66 @@
+name: CI
+
+# The release workflow gates a tag, which is late: a broken commit sits green until
+# somebody decides to publish. This runs the same checks on the way in.
+#
+# push is scoped to main rather than all branches so that a branch pushed as part of a
+# pull request is not checked twice.
+#
+# No actions/checkout, deliberately -- same as the letsvisit and charts workflows. The
+# runner image is ubuntu:22.04 whose `nodejs` package is Node 12, and actions/checkout@v4
+# is built with ES2022 static initialiser blocks, so it dies with
+# `SyntaxError: Unexpected token '{'` before running. Cloning with git directly avoids JS
+# actions entirely. This repo is public, so the clone needs no credential at all.
+#
+# `${{ }}` values are passed through `env:` and referenced as quoted shell variables: a
+# ref name is attacker-influenced by anyone who can push a branch or open a PR, and
+# expanding one straight into `run:` is a shell-injection vector.
+on:
+  push:
+    branches: [main]
+  pull_request:
+
+# A rapid series of pushes only needs the last one checked.
+concurrency:
+  group: ci-${{ github.ref }}
+  cancel-in-progress: true
+
+env:
+  REPO_URL: https://git.ryuvia.com/niklas/terdut-server.git
+
+jobs:
+  test:
+    runs-on: ubuntu-latest
+    container:
+      # Runs inside the toolchain image rather than installing Go per job. Note this puts
+      # the job on the dind bridge, which cannot reach github.com or get.helm.sh --
+      # proxy.golang.org and git.ryuvia.com are reachable, which is all this job needs.
+      image: golang:1.26.6-bookworm
+      # act_runner destroys a job's own volumes when it finishes, so without these every
+      # run re-downloads the whole module graph. The names must appear in the runner's
+      # container.valid_volumes allowlist (charts/act-runner in the k8s repo); unlisted
+      # volumes are dropped silently, so a workflow that looks correct can still be
+      # running uncached.
+      volumes:
+        - go-mod-cache:/go/pkg/mod
+        - go-build-cache:/root/.cache/go-build
+        - gobin-cache:/go/bin
+    steps:
+      - name: Checkout
+        env:
+          REF_NAME: ${{ github.ref_name }}
+          HEAD_SHA: ${{ github.event.pull_request.head.sha }}
+        run: |
+          if [ -n "$HEAD_SHA" ]; then
+            # A pull_request ref_name is "<n>/merge", which is not a fetchable branch.
+            git clone "$REPO_URL" .
+            git checkout -q "$HEAD_SHA"
+          else
+            git clone --depth=1 --branch "$REF_NAME" "$REPO_URL" .
+          fi
+
+      - name: Vet
+        run: go vet ./...
+
+      - name: Test
+        run: go test ./...