Stop tracking .env

It has been in the repository since the initial commit carrying the live
database host, user and password. Removing it from the index stops that
getting worse; the credentials are still in history and should be
rotated, which needs coordinating with everything that reads them.

Deployments should pass configuration as container environment rather
than shipping a file — a file on disk is one `git add -f` away from
being committed again.

Also closes the last untested path: a shopper registration published
over the broker rather than posted over HTTP. All three MQTT topics —
order, customer and health — have now been fired against the live
Mosquitto instance and acknowledged.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

scratch/mqttpub/main.go

@@ -118,5 +118,31 @@ func main() {
 		log.Fatal("publish health:", t.Error())
 	}
 	fmt.Println("\npublished a heartbeat to", healthTopic)
-	time.Sleep(2 * time.Second)
+
+	// The registration uplink. Tested over HTTP early on; this is the same
+	// service reached over the broker, which is the path a real till uses.
+	custBatch, _ := json.Marshal(map[string]any{
+		"schema": 1, "batch_id": "batch-cust-mqtt-0001",
+		"store_id": locationID, "terminal_id": terminalID,
+		"customers": []map[string]any{{
+			"id":                     "3d7a0000-0000-4000-8000-000000000001",
+			"mobile":                 "9840077777",
+			"name":                   "MQTT Probe Shopper",
+			"registered_at":          time.Now().UTC().Format(time.RFC3339),
+			"registered_by_terminal": terminalID,
+		}},
+	})
+	custTopic := fmt.Sprintf("nearle/pos/%s/%s/customer", locationID, terminalID)
+	if t := client.Publish(custTopic, 1, false, custBatch); t.Wait() && t.Error() != nil {
+		log.Fatal("publish customer:", t.Error())
+	}
+	fmt.Println("published a registration to", custTopic)
+
+	select {
+	case payload := <-acks:
+		fmt.Println("  registration ACK:", string(payload))
+	case <-time.After(20 * time.Second):
+		fmt.Println("  NO ACK for the registration")
+		os.Exit(1)
+	}
 }