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.API/Models/Printing/PrintAgentDtos.cs

@@ -0,0 +1,26 @@
+namespace Meezi.API.Models.Printing;
+
+public record PrintAgentDeviceDto(
+    string Id,
+    string SystemName,
+    string DisplayName,
+    string Kind,
+    DateTime LastSeenAt);
+
+public record PrintAgentDto(
+    string Id,
+    string Name,
+    string? BranchId,
+    bool Online,
+    bool Paired,
+    DateTime? LastSeenAt,
+    DateTime CreatedAt,
+    IReadOnlyList<PrintAgentDeviceDto> Devices);
+
+public record CreatePairingCodeRequest(string? Name, string? BranchId);
+
+public record PairingCodeResponse(string AgentId, string Code, DateTime ExpiresAt);
+
+public record ClaimAgentRequest(string Code, string? Name, string? MachineName);
+
+public record ClaimAgentResponse(string AgentId, string Token, string CafeId, string AgentName);