feat(print): route print jobs through a local agent, fall back to TCP

Phase 2. NetworkPrinterService now builds the ESC/POS bytes (as before) and
dispatches them via a new router: if the receipt/kitchen/station printer is mapped
to a PrintDevice whose agent is online, the bytes are sent to that agent over the
hub and we await its ack; otherwise it falls back to a direct TCP connection (raw
IP), so existing on-prem/reachable printers keep working unchanged. Adds nullable
mapping columns Branch.ReceiptPrintDeviceId / KitchenPrintDeviceId and
KitchenStation.PrintDeviceId (additive migration), plus TestPrintDeviceAsync for
testing an agent printer. The cloud can now reach LAN/USB printers it never could.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/Meezi.Core/Entities/Branch.cs

@@ -18,6 +18,11 @@ public class Branch : TenantEntity
     public int? ReceiptPrinterPort { get; set; }
     public string? KitchenPrinterIp { get; set; }
     public int? KitchenPrinterPort { get; set; }
+
+    /// <summary>Optional <see cref="PrintDevice"/> to route through a local print agent
+    /// (preferred over the raw IP when its agent is online). Cloud-hosted cafés use this.</summary>
+    public string? ReceiptPrintDeviceId { get; set; }
+    public string? KitchenPrintDeviceId { get; set; }
     public int PaperWidthMm { get; set; } = 80;
     public bool AutoCutEnabled { get; set; } = true;
     public string? ReceiptHeader { get; set; }