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/export_test.go

@@ -1,5 +1,13 @@
 package server
 
+import "github.com/go-chi/chi/v5"
+
+// Router exposes the router to the external tests, which add routes
+// of their own to it after SetupRoutes.
+func (s *Server) Router() *chi.Mux {
+	return s.router
+}
+
 // MaxRequestBodyBytes exposes the router-wide body limit to the
 // external tests.
 const MaxRequestBodyBytes = maxRequestBodyBytes