feat(server): add observability and operator controls (#16)

src/FinalFactory.Rendezvous.Server/State/EphemeralStateContracts.cs

@@ -297,6 +297,7 @@ internal sealed record StoredJoinAttempt
     public required SecretFingerprint ConnectionTicketFingerprint { get; init; }
     public NetworkEndpoint? DedicatedFallback { get; init; }
     public required DateTimeOffset ExpiresAt { get; init; }
+    public required TimeSpan CreatedAtMonotonic { get; init; }
     public required DateTimeOffset ConnectionTicketExpiresAt { get; init; }
     public AttemptEndpointBinding? HostEndpoint { get; init; }
     public AttemptEndpointBinding? ClientEndpoint { get; init; }