fix: replace time.Tick with NewTicker in prune goroutine for graceful shutdown (#593)

## Summary

Replace `time.Tick()` with `time.NewTicker()` in the auto-prune
goroutine so it stops cleanly during graceful shutdown.

## Problem

`time.Tick` creates a ticker that can never be garbage collected or
stopped. While the prune goroutine runs for the process lifetime, it
won't stop during graceful shutdown — the goroutine leaks past the
shutdown sequence.

## Fix

- Create a `time.NewTicker` and a done channel
- Use `select` to listen on both the ticker and done channel
- Stop the ticker and close the done channel in the shutdown path (after
`poller.Stop()`)
- Pattern matches the existing `StartEvictionTicker()` approach

## Testing

- `go build ./...` — compiles cleanly
- `go test ./...` — all tests pass

Fixes #377

Co-authored-by: you <you@example.com>

cmd/server/main.go

@@ -224,8 +224,15 @@ func main() {
 	defer stopEviction()
 
 	// Auto-prune old packets if retention.packetDays is configured
+	var stopPrune func()
 	if cfg.Retention != nil && cfg.Retention.PacketDays > 0 {
 		days := cfg.Retention.PacketDays
+		pruneTicker := time.NewTicker(24 * time.Hour)
+		pruneDone := make(chan struct{})
+		stopPrune = func() {
+			pruneTicker.Stop()
+			close(pruneDone)
+		}
 		go func() {
 			time.Sleep(1 * time.Minute)
 			if n, err := database.PruneOldPackets(days); err != nil {
@@ -233,11 +240,16 @@ func main() {
 			} else {
 				log.Printf("[prune] deleted %d transmissions older than %d days", n, days)
 			}
-			for range time.Tick(24 * time.Hour) {
-				if n, err := database.PruneOldPackets(days); err != nil {
-					log.Printf("[prune] error: %v", err)
-				} else {
-					log.Printf("[prune] deleted %d transmissions older than %d days", n, days)
+			for {
+				select {
+				case <-pruneTicker.C:
+					if n, err := database.PruneOldPackets(days); err != nil {
+						log.Printf("[prune] error: %v", err)
+					} else {
+						log.Printf("[prune] deleted %d transmissions older than %d days", n, days)
+					}
+				case <-pruneDone:
+					return
 				}
 			}
 		}()
@@ -262,6 +274,11 @@ func main() {
 		// 1. Stop accepting new WebSocket/poll data
 		poller.Stop()
 
+		// 1b. Stop auto-prune ticker
+		if stopPrune != nil {
+			stopPrune()
+		}
+
 		// 2. Gracefully drain HTTP connections (up to 15s)
 		ctx, cancel := context.WithTimeout(context.Background(), 15*time.Second)
 		defer cancel()