Report handler panics to Sentry when SENTRY_DSN is set (closes #95)

With SENTRY_DSN set, the server initialises sentry-go with the release
netwatch-server-<version>, adds the sentryhttp middleware with Repanic
as the last router-wide middleware, after the timeout, and flushes
Sentry for 2 seconds on shutdown. A DSN Sentry refuses stops the start
with an error naming SENTRY_DSN. With it empty, nothing is set up.

The metrics middleware stays on the matched routes only, so it runs
inside the Sentry middleware rather than before it.

Model: opus-5-5

backend/cmd/netwatch-server/main_test.go

@@ -115,6 +115,28 @@ func TestMalformedConfigFileStopsTheStart(t *testing.T) {
 	}
 }
 
+// TestRefusedSentryDSNStopsTheStart: a SENTRY_DSN that Sentry refuses
+// stops the start, and the error, naming SENTRY_DSN, is logged as JSON.
+func TestRefusedSentryDSNStopsTheStart(t *testing.T) {
+	t.Setenv("SENTRY_DSN", "not-a-dsn")
+
+	ctx, cancel := context.WithTimeout(t.Context(), childTimeout)
+	defer cancel()
+
+	child, stdout, stderr := startServer(ctx, t, t.TempDir(), freePort(ctx, t))
+
+	err := child.Wait()
+	if child.ProcessState.ExitCode() != 1 {
+		t.Fatalf("server exit = %v, want exit status 1", err)
+	}
+
+	requireJSONLines(t, stdout, stderr)
+
+	if !strings.Contains(stdout.String(), "SENTRY_DSN") {
+		t.Fatalf("no error naming SENTRY_DSN in stdout:\n%s", stdout)
+	}
+}
+
 // startServer runs main() in a child process listening on
 // 127.0.0.1:port, with home as its HOME and working directory and its
 // data directory in home, so it touches nothing outside home. Its