Bound each shutdown stage by the stop deadline (closes #100)

Shutdown passes its context to every stage: the streamer, the four
handlers, the WHOIS fetcher, the database maintainer and the HTTP server
each stop waiting when it ends. A handler then skips its last write and
logs how many updates it did not write, and the HTTP server closes the
connections still open. The database is checkpointed and closed last,
and left open once the deadline has passed, since a write may still
hold it. The stop hook gives Shutdown 8 seconds and fx 9, inside the 10
seconds upaas and docker stop allow, so a stop signal exits 0.

Model: opus-5-5

internal/routewatch/asnfetcher.go

@@ -43,7 +43,7 @@ type ASNFetcher struct {
 	logger         *slog.Logger
 	immediateQueue chan int
 	stopCh         chan struct{}
-	wg             sync.WaitGroup
+	done           chan struct{} // closed when the fetcher goroutine returns
 
 	// fetchMu ensures only one fetch runs at a time
 	fetchMu sync.Mutex
@@ -67,6 +67,7 @@ func NewASNFetcher(db database.Store, logger *slog.Logger) *ASNFetcher {
 		logger:          logger.With("component", "asn_fetcher"),
 		immediateQueue:  make(chan int, immediateQueueSize),
 		stopCh:          make(chan struct{}),
+		done:            make(chan struct{}),
 		currentInterval: baseInterval,
 		successTimes:    make([]time.Time, 0),
 		errorTimes:      make([]time.Time, 0),
@@ -76,8 +77,6 @@ func NewASNFetcher(db database.Store, logger *slog.Logger) *ASNFetcher {
 // Start begins the background ASN fetcher goroutine. It runs until Stop:
 // the end of ctx does not stop it.
 func (f *ASNFetcher) Start(ctx context.Context) {
-	f.wg.Add(1)
-
 	go f.run(context.WithoutCancel(ctx))
 
 	f.logger.Info("ASN fetcher started",
@@ -87,11 +86,17 @@ func (f *ASNFetcher) Start(ctx context.Context) {
 	)
 }
 
-// Stop gracefully shuts down the fetcher.
-func (f *ASNFetcher) Stop() {
+// Stop stops the fetcher and waits, until ctx ends, for a lookup in progress to
+// finish.
+func (f *ASNFetcher) Stop(ctx context.Context) {
 	close(f.stopCh)
-	f.wg.Wait()
-	f.logger.Info("ASN fetcher stopped")
+
+	select {
+	case <-f.done:
+		f.logger.Info("ASN fetcher stopped")
+	case <-ctx.Done():
+		f.logger.Warn("Stop deadline passed before the ASN fetcher stopped")
+	}
 }
 
 // QueueImmediate queues an ASN for immediate WHOIS lookup.
@@ -197,7 +202,7 @@ func (f *ASNFetcher) recordFailure() {
 
 // run is the main background loop.
 func (f *ASNFetcher) run(ctx context.Context) {
-	defer f.wg.Done()
+	defer close(f.done)
 
 	timer := time.NewTimer(f.getInterval())
 	defer timer.Stop()