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/server.go

@@ -7,8 +7,10 @@ package server
 
 import (
 	"context"
+	"fmt"
 	"log/slog"
 	"net/http"
+	"time"
 
 	"sneak.berlin/go/netwatch/internal/config"
 	"sneak.berlin/go/netwatch/internal/globals"
@@ -16,10 +18,15 @@ import (
 	"sneak.berlin/go/netwatch/internal/logger"
 	"sneak.berlin/go/netwatch/internal/middleware"
 
+	"github.com/getsentry/sentry-go"
 	"github.com/go-chi/chi/v5"
 	"go.uber.org/fx"
 )
 
+// sentryFlushTimeout is how long shutdown waits for Sentry to send
+// what it still holds.
+const sentryFlushTimeout = 2 * time.Second
+
 // Params defines the dependencies for Server.
 type Params struct {
 	fx.In
@@ -56,6 +63,11 @@ func New(
 	s.log = params.Logger.Get()
 	s.shutdowner = params.Shutdowner
 
+	err := s.enableSentry()
+	if err != nil {
+		return nil, err
+	}
+
 	lc.Append(fx.Hook{
 		OnStart: func(_ context.Context) error {
 			// Build the router and http.Server synchronously
@@ -87,10 +99,37 @@ func (s *Server) ServeHTTP(
 	s.router.ServeHTTP(w, r)
 }
 
+// enableSentry sets Sentry up when SENTRY_DSN is set, so that
+// SetupRoutes can report panics to it. With SENTRY_DSN empty it does
+// nothing. A DSN Sentry refuses stops the start.
+func (s *Server) enableSentry() error {
+	if s.params.Config.SentryDSN == "" {
+		return nil
+	}
+
+	err := sentry.Init(sentry.ClientOptions{
+		Dsn:     s.params.Config.SentryDSN,
+		Release: s.params.Globals.Appname + "-" + s.params.Globals.Version,
+	})
+	if err != nil {
+		return fmt.Errorf("SENTRY_DSN: %w", err)
+	}
+
+	s.log.Info("sentry error reporting activated")
+
+	return nil
+}
+
 // shutdown gracefully stops the HTTP server within the
-// deadline of the context fx provides for OnStop.
+// deadline of the context fx provides for OnStop, then gives
+// Sentry, if set up, time to send what it still holds.
 func (s *Server) shutdown(ctx context.Context) error {
 	err := s.httpServer.Shutdown(ctx)
+
+	if s.params.Config.SentryDSN != "" {
+		sentry.Flush(sentryFlushTimeout)
+	}
+
 	if err != nil {
 		s.log.Error("server clean shutdown failed", "error", err)