server: report HTTP handler panics to Sentry (closes #107)

DNSWATCHER_SENTRY_DSN was read but never used. This ports the Sentry
integration from gohttpserver with sentry-go v0.49.0. The server's start
hook calls sentry.Init when the DSN is set; a DSN Sentry cannot parse
fails the hook, so startup stops with the parse error. sentryhttp, with
Repanic, reports handler panics and passes them on to chi's Recoverer.
Shutdown sends queued reports once the HTTP server has stopped. The
client uses the older transport (DisableTelemetryBuffer): with the
default one, Flush can return before sending a report made just before
it. The DSN is checked at server start, not in config, so the config
test is unchanged. sentry-go raises several golang.org/x modules.

Model: opus-5-5

internal/server/export_test.go

@@ -3,6 +3,8 @@ package server
 import (
 	"net/http"
 	"time"
+
+	"github.com/go-chi/chi/v5"
 )
 
 // RequestTimeout exports the handler execution budget applied by
@@ -22,3 +24,15 @@ func SetListenPort(s *Server, port int) {
 func HTTPServerOf(s *Server) *http.Server {
 	return s.httpServer
 }
+
+// EnableSentry runs the Sentry setup that the start hook runs, without
+// starting the HTTP server.
+func EnableSentry(s *Server) error {
+	return s.enableSentry()
+}
+
+// RouterOf returns the router SetupRoutes built, so a test can add a
+// route that panics.
+func RouterOf(s *Server) *chi.Mux {
+	return s.router
+}