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.

Dockerfile

@@ -1,10 +1,19 @@
-FROM golang:1.25-alpine AS builder
+# --platform=$BUILDPLATFORM pins the builder to the machine doing the building, so a
+# multi-arch build compiles both targets natively instead of running an emulated arm64
+# toolchain under QEMU. Go cross-compiles from TARGETOS/TARGETARCH, which BuildKit fills
+# in per platform. The CI runner has no binfmt registration and no way to get one (the
+# JS action that used to install it cannot run there), so this is not just an
+# optimisation -- it is what makes the arm64 image buildable at all.
+FROM --platform=$BUILDPLATFORM golang:1.25-alpine AS builder
 WORKDIR /src
 COPY go.mod go.sum ./
 RUN go mod download
 COPY . .
 ARG VERSION=dev
-RUN CGO_ENABLED=0 GOOS=linux go build -ldflags="-w -s -X main.version=${VERSION}" -o /terdut ./cmd/terdut
+ARG TARGETOS
+ARG TARGETARCH
+RUN CGO_ENABLED=0 GOOS=${TARGETOS} GOARCH=${TARGETARCH} \
+    go build -ldflags="-w -s -X main.version=${VERSION}" -o /terdut ./cmd/terdut
 
 FROM scratch
 COPY --from=builder /terdut /terdut