Let a host marked for the relay actually be reached through it

The relay checkbox said it was not wired up, and the plan said the server
half was built and shipped. It was not: there was ADR 0004, the contract
records, RelayOptions and a CHECK constraint, and RelayOptions said in its
own remarks that there was no endpoint and no dialler. So both halves are
built here, as step 3 of docs/reaching-a-host-you-cannot-dial.md.

Server, in Features/Relay:

- POST /api/v1/relay/tickets makes every decision: the host, Connect on
  its vault, the relay switched on, and the stored name resolved once and
  judged by RelayAddressPolicy. Loopback, link-local, 0.0.0.0/8,
  multicast and the metadata endpoints are refused with no configuration
  escape, with IPv4-mapped and NAT64 spellings unwrapped first; then the
  operator's denials, allow-list and private-network switch. What passes
  goes into a Data Protection ticket, and nothing resolves the name again.
  Every target refusal is the same 403, so the endpoint cannot be used to
  map the operator's DNS; the real reason is logged.
- GET /api/v1/relay/connect takes the ticket as a second subprotocol,
  checks its expiry against TimeProvider and spends it with a unique
  insert into relay_ticket_use before accepting the upgrade. After the
  upgrade a refusal is a close code: 4502 for a host the server could not
  reach, 4429 for a limit, 4408 idle, 4409 lifetime, 1001 after a drain.
- relay_session is the audit row, written from short-lived scopes.

Client:

- RelayBridge is a loopback SOCKS5 listener that accepts once and pipes
  to the relay socket. It rides on the new SshLoopbackProxy.Lifetime, so
  the connection disposes it on every way a session ends and a failed
  connect disposes it before throwing.
- ServerRelay, exposed as IVaultServer.Relay from /api/v1/meta, asks for
  the ticket, opens the socket, starts the bridge and turns every refusal
  into a sentence.
- VaultViewModel and TransfersViewModel reach a relayed host through it,
  never fall back to dialling it directly, and replace SSH.NET's transport
  error with the relay's own reason when it gave one.

Neither side ever cancels a WebSocket operation. Cancelling one aborts
the socket, and an aborted socket sends no close frame, which is the only
way the client learns the server could not reach the host.
AnIdleSession_IsClosedWithTheIdleCode asserts that the code arrives.

Three departures from ADR 0004, recorded in it: awaited copy loops rather
than Pipelines (same backpressure, two 32 KiB buffers), no ?ticket=
fallback, and no device id in the ticket.

Tests: API 245, shell 582 (5 skipped), layout 170, SSH 110 including a
real login through the bridge, API client 30, contracts 25. Not run: the
Android build (its change is copy only) and the Alpine layout oracle.

src/DodoSSH.Api/Features/Relay/RelaySessionRegistry.cs

@@ -0,0 +1,111 @@
+using DodoSSH.Api.Setup;
+using Microsoft.Extensions.Options;
+
+namespace DodoSSH.Api.Features.Relay;
+
+/// <summary>
+/// Counts the relay sessions open on this node, and refuses the ones over ADR 0004's limits.
+/// </summary>
+/// <remarks>
+/// <para>
+/// Per node, and so per user <em>per node</em>: a user behind a round-robin balancer with three nodes
+/// could hold three times the per-user cap. That is accepted rather than fixed, because fixing it means
+/// shared state on the hot path of every connect, and the cap exists to stop one account exhausting a
+/// node — which a per-node count does exactly.
+/// </para>
+/// <para>
+/// A singleton, because a session outlives the request that opened it. It holds counts and nothing else;
+/// the sockets belong to the handlers that opened them.
+/// </para>
+/// </remarks>
+internal sealed class RelaySessionRegistry(IOptions<RelayOptions> options)
+{
+    private readonly Lock gate = new();
+    private readonly Dictionary<Guid, int> perUser = [];
+    private int total;
+
+    /// <summary>How many sessions are open on this node.</summary>
+    internal int Count
+    {
+        get
+        {
+            lock (gate)
+            {
+                return total;
+            }
+        }
+    }
+
+    /// <summary>Whether a session for this user would be admitted now.</summary>
+    /// <remarks>
+    /// Advisory, for the ticket endpoint: it lets a caller already at the cap hear so as an HTTP status
+    /// before it has spent a ticket, but only <see cref="TryAdmit"/> decides, since the answer here can
+    /// be stale by the time the socket arrives — possibly at another node.
+    /// </remarks>
+    internal bool HasRoomFor(Guid userId)
+    {
+        var settings = options.Value;
+
+        lock (gate)
+        {
+            return total < settings.MaxConcurrentSessionsTotal
+                && perUser.GetValueOrDefault(userId) < settings.MaxConcurrentSessionsPerUser;
+        }
+    }
+
+    /// <summary>Admits a session, or answers null when a limit is met.</summary>
+    /// <returns>A lease that gives the place back when disposed.</returns>
+    internal Lease? TryAdmit(Guid userId)
+    {
+        var settings = options.Value;
+
+        lock (gate)
+        {
+            var held = perUser.GetValueOrDefault(userId);
+
+            if (total >= settings.MaxConcurrentSessionsTotal || held >= settings.MaxConcurrentSessionsPerUser)
+            {
+                return null;
+            }
+
+            perUser[userId] = held + 1;
+            total++;
+
+            return new Lease(this, userId);
+        }
+    }
+
+    private void Release(Guid userId)
+    {
+        lock (gate)
+        {
+            total--;
+
+            // Removed at zero rather than left behind, so the dictionary is the size of the users with a
+            // session open and not of every user who ever had one.
+            if (perUser[userId] <= 1)
+            {
+                perUser.Remove(userId);
+            }
+            else
+            {
+                perUser[userId]--;
+            }
+        }
+    }
+
+    /// <summary>One admitted session's place. Idempotent to dispose.</summary>
+    internal sealed class Lease(RelaySessionRegistry registry, Guid userId) : IDisposable
+    {
+        private int released;
+
+        /// <inheritdoc />
+        public void Dispose()
+        {
+            if (Interlocked.Exchange(ref released, 1) == 0)
+            {
+                registry.Release(userId);
+            }
+        }
+    }
+}