Add backend with buffered zstd-compressed report storage

Introduce the Go backend (netwatch-server) with an HTTP API that
accepts telemetry reports and persists them as zstd-compressed JSONL
files. Reports are buffered in memory and flushed to disk when the
buffer reaches 10 MiB or every 60 seconds.

backend/Dockerfile

@@ -0,0 +1,22 @@
+# golang:1.24-alpine (2026-02-26)
+FROM golang:1.24-alpine@sha256:8bee1901f1e530bfb4a7850aa7a479d17ae3a18beb6e09064ed54cfd245b7191 AS builder
+
+RUN apk add --no-cache git make gcc musl-dev
+
+WORKDIR /src
+COPY go.mod go.sum ./
+RUN go mod download
+COPY . .
+
+RUN make check
+RUN make build
+
+# alpine:3.21 (2026-02-26)
+FROM alpine:3.21@sha256:c3f8e73fdb79deaebaa2037150150191b9dcbfba68b4a46d70103204c53f4709
+
+RUN apk add --no-cache ca-certificates
+COPY --from=builder /src/netwatch-server /usr/local/bin/netwatch-server
+COPY --from=builder /src /src
+
+EXPOSE 8080
+ENTRYPOINT ["netwatch-server"]