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.

README.md

@@ -17,7 +17,7 @@ Incident management server for teams using Prometheus Alertmanager.
 **Prerequisites:** Go 1.21+
 
 ```bash
-git clone https://github.com/yeniklas/terdut-server
+git clone https://git.ryuvia.com/niklas/terdut-server
 cd terdut-server
 go run ./cmd/terdut
 ```
@@ -52,11 +52,12 @@ docker run -p 8080:8080 -v $(pwd)/data:/data \
 
 ### Kubernetes
 
-A Helm chart is published from this repository:
+A Helm chart is published from this repository as an OCI artifact, versioned in lockstep
+with the app — chart `x.y.z` is always app `vx.y.z`:
 
 ```bash
-helm repo add terdut-server https://yeniklas.github.io/terdut-server
-helm upgrade --install terdut-server terdut-server/terdut-server \
+helm upgrade --install terdut-server oci://git.ryuvia.com/niklas/terdut-server \
+  --version 0.9.0 \
   --namespace terdut-server --create-namespace \
   --set networking.hostname=terdut.example.com
 ```