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/Makefile

@@ -0,0 +1,53 @@
+UNAME_S := $(shell uname -s)
+VERSION := $(shell git describe --always --dirty)
+BUILDARCH := $(shell uname -m)
+BINARY := netwatch-server
+
+GOLDFLAGS += -X main.Version=$(VERSION)
+GOLDFLAGS += -X main.Buildarch=$(BUILDARCH)
+
+ifeq ($(UNAME_S),Darwin)
+	GOFLAGS := -ldflags "$(GOLDFLAGS)"
+else
+	GOFLAGS = -ldflags "-linkmode external -extldflags -static $(GOLDFLAGS)"
+endif
+
+.PHONY: all build test lint fmt fmt-check check docker hooks run clean
+
+all: build
+
+build: ./$(BINARY)
+
+./$(BINARY): $(shell find . -name '*.go' -type f) go.mod go.sum
+	go build -o $@ $(GOFLAGS) ./cmd/netwatch-server/
+
+test:
+	timeout 30 go test ./...
+
+lint:
+	golangci-lint run ./...
+
+fmt:
+	go fmt ./...
+
+fmt-check:
+	@test -z "$$(gofmt -l .)" || \
+		(echo "Files not formatted:"; gofmt -l .; exit 1)
+
+check: test lint fmt-check
+
+docker:
+	timeout 300 docker build -t netwatch-server .
+
+hooks:
+	@printf '#!/bin/sh\ncd backend && make check\n' > \
+		$$(git rev-parse --show-toplevel)/.git/hooks/pre-commit
+	@chmod +x \
+		$$(git rev-parse --show-toplevel)/.git/hooks/pre-commit
+	@echo "Pre-commit hook installed"
+
+run: build
+	./$(BINARY)
+
+clean:
+	rm -f ./$(BINARY)