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/Services/Printing/PrintAgentRegistry.cs

@@ -0,0 +1,90 @@
+using System.Collections.Concurrent;
+using Microsoft.AspNetCore.SignalR;
+using Meezi.API.Hubs;
+
+namespace Meezi.API.Services.Printing;
+
+public record PrintJobRequest(string PrinterSystemName, byte[] Payload);
+public record PrintJobOutcome(bool Success, string? Error);
+
+/// <summary>
+/// Tracks which print agents are currently connected (by SignalR connection) and
+/// dispatches print jobs to them, awaiting the agent's acknowledgement. In-memory:
+/// a dropped process simply means agents reconnect and re-register.
+/// </summary>
+public interface IPrintAgentRegistry
+{
+    void Register(string connectionId, string agentId, string cafeId);
+    void Unregister(string connectionId);
+    (string AgentId, string CafeId)? Resolve(string connectionId);
+    bool IsOnline(string agentId);
+    IReadOnlySet<string> OnlineAgentIds();
+    Task<PrintJobOutcome> SendJobAsync(string agentId, PrintJobRequest job, CancellationToken ct = default);
+    void CompleteJob(string jobId, bool success, string? error);
+}
+
+public class PrintAgentRegistry : IPrintAgentRegistry
+{
+    private readonly IHubContext<PrintAgentHub> _hub;
+    private readonly ConcurrentDictionary<string, (string AgentId, string CafeId)> _byConnection = new();
+    private readonly ConcurrentDictionary<string, string> _agentConnection = new(); // agentId -> connectionId
+    private readonly ConcurrentDictionary<string, TaskCompletionSource<PrintJobOutcome>> _pending = new();
+
+    public PrintAgentRegistry(IHubContext<PrintAgentHub> hub) => _hub = hub;
+
+    public void Register(string connectionId, string agentId, string cafeId)
+    {
+        _byConnection[connectionId] = (agentId, cafeId);
+        _agentConnection[agentId] = connectionId;
+    }
+
+    public void Unregister(string connectionId)
+    {
+        if (!_byConnection.TryRemove(connectionId, out var info)) return;
+        // Only drop the agent→connection mapping if it still points at this socket
+        // (a fast reconnect may already have replaced it with a newer one).
+        if (_agentConnection.TryGetValue(info.AgentId, out var current) && current == connectionId)
+            _agentConnection.TryRemove(info.AgentId, out _);
+    }
+
+    public (string AgentId, string CafeId)? Resolve(string connectionId) =>
+        _byConnection.TryGetValue(connectionId, out var info) ? info : null;
+
+    public bool IsOnline(string agentId) => _agentConnection.ContainsKey(agentId);
+
+    public IReadOnlySet<string> OnlineAgentIds() => _agentConnection.Keys.ToHashSet();
+
+    public async Task<PrintJobOutcome> SendJobAsync(string agentId, PrintJobRequest job, CancellationToken ct = default)
+    {
+        if (!_agentConnection.TryGetValue(agentId, out var connectionId))
+            return new PrintJobOutcome(false, "AGENT_OFFLINE");
+
+        var jobId = Guid.NewGuid().ToString("N");
+        var tcs = new TaskCompletionSource<PrintJobOutcome>(TaskCreationOptions.RunContinuationsAsynchronously);
+        _pending[jobId] = tcs;
+        try
+        {
+            await _hub.Clients.Client(connectionId).SendAsync(
+                "PrintJob", jobId, job.PrinterSystemName, Convert.ToBase64String(job.Payload), ct);
+
+            using var timeout = CancellationTokenSource.CreateLinkedTokenSource(ct);
+            timeout.CancelAfter(TimeSpan.FromSeconds(20));
+            using var reg = timeout.Token.Register(() => tcs.TrySetResult(new PrintJobOutcome(false, "TIMEOUT")));
+            return await tcs.Task;
+        }
+        catch (Exception ex)
+        {
+            return new PrintJobOutcome(false, ex.Message);
+        }
+        finally
+        {
+            _pending.TryRemove(jobId, out _);
+        }
+    }
+
+    public void CompleteJob(string jobId, bool success, string? error)
+    {
+        if (_pending.TryGetValue(jobId, out var tcs))
+            tcs.TrySetResult(new PrintJobOutcome(success, error));
+    }
+}