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/internal/server/routes.go

@@ -3,6 +3,7 @@ package server
 import (
 	"time"
 
+	sentryhttp "github.com/getsentry/sentry-go/http"
 	"github.com/go-chi/chi/v5"
 	"github.com/go-chi/chi/v5/middleware"
 	"github.com/prometheus/client_golang/prometheus"
@@ -32,6 +33,12 @@ func (s *Server) SetupRoutes() {
 	s.router.Use(s.mw.MaxBodyBytes(maxRequestBodyBytes))
 	s.router.Use(middleware.Timeout(requestTimeout))
 
+	// Sentry reports a panic, then panics again, so that s.mw.Recoverer
+	// still answers 500.
+	if s.params.Config.SentryDSN != "" {
+		s.router.Use(sentryhttp.New(sentryhttp.Options{Repanic: true}).Handle)
+	}
+
 	// The metrics go in a registry of this server's own, not in
 	// Prometheus' default one, which takes them only once per process.
 	registry := prometheus.NewRegistry()