feat(print): cloud↔local print-agent foundation (hub, pairing, registry)

First phase of auto-discovered printing for cloud-hosted cafés whose printers are
on the local network (the cloud can't reach a LAN/USB printer directly). Adds:
- PrintAgent + PrintDevice entities (+ additive migration) — a per-café local
  bridge and the printers it reports.
- PrintAgentHub (/hubs/print-agent): agents connect outbound, authenticated by a
  token in access_token (not the user JWT); ReportPrinters upserts devices,
  PrintJob is pushed to the agent, JobResult/Heartbeat come back.
- PrintAgentRegistry (singleton): tracks connected agents and dispatches a job to
  one, awaiting its ack with a timeout.
- Pairing: POST /cafes/{id}/print-agents/pairing-code (ManagePrintSettings) issues
  a short one-time code; anonymous POST /print-agent/claim redeems it for a
  long-lived token (only its SHA-256 hash is stored). List + revoke endpoints,
  online status from the registry.

Inert until Phase 2 routes jobs through it and the agent app (Phase 3) connects.

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

src/Meezi.Core/Entities/PrintAgent.cs

@@ -0,0 +1,29 @@
+namespace Meezi.Core.Entities;
+
+/// <summary>
+/// A local print bridge installed on a café PC. It connects outbound to the cloud
+/// over SignalR (authenticated by its token), reports the printers it can see, and
+/// relays print jobs to them — so the cloud can reach LAN/USB printers it could
+/// never connect to directly.
+/// </summary>
+public class PrintAgent : TenantEntity
+{
+    public string? BranchId { get; set; }
+    public string Name { get; set; } = string.Empty;
+
+    /// <summary>Short one-time code shown in the dashboard; the agent exchanges it for a token.</summary>
+    public string? PairingCode { get; set; }
+    public DateTime? PairingCodeExpiresAt { get; set; }
+
+    /// <summary>SHA-256 (hex) of the long-lived agent token. Null until the agent is paired.</summary>
+    public string? TokenHash { get; set; }
+
+    /// <summary>Last time the agent connected or sent a heartbeat (UTC).</summary>
+    public DateTime? LastSeenAt { get; set; }
+
+    public bool Revoked { get; set; }
+
+    public Cafe Cafe { get; set; } = null!;
+    public Branch? Branch { get; set; }
+    public ICollection<PrintDevice> Devices { get; set; } = [];
+}

src/Meezi.Core/Entities/PrintDevice.cs

@@ -0,0 +1,18 @@
+namespace Meezi.Core.Entities;
+
+/// <summary>A printer discovered and reported by a <see cref="PrintAgent"/>.</summary>
+public class PrintDevice : TenantEntity
+{
+    public string AgentId { get; set; } = string.Empty;
+
+    /// <summary>Stable identifier the agent uses to print (OS printer name, or "ip:port").</summary>
+    public string SystemName { get; set; } = string.Empty;
+    public string DisplayName { get; set; } = string.Empty;
+
+    /// <summary>"usb" | "network" | "other".</summary>
+    public string Kind { get; set; } = "other";
+
+    public DateTime LastSeenAt { get; set; } = DateTime.UtcNow;
+
+    public PrintAgent Agent { get; set; } = null!;
+}