feat: add stats helper service

Periodically snapshots Gitea instance metrics from a read-only MySQL
user and writes stats.json into Gitea custom/public/assets for the
home page to fetch same-origin. Config via env vars only.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

stats/Dockerfile

@@ -0,0 +1,11 @@
+# build
+FROM golang:1.23-alpine AS build
+WORKDIR /src
+COPY go.mod ./
+COPY main.go ./
+RUN go mod tidy && CGO_ENABLED=0 GOOS=linux go build -trimpath -ldflags "-s -w" -o /stats .
+
+# runtime (minimal)
+FROM scratch
+COPY --from=build /stats /stats
+ENTRYPOINT ["/stats"]