Add user identity system with channel pairing and web /link command

Introduce Luna.Identity project with user identity service, pairing code
generation, and channel linking. Channels (Telegram, CLI) now verify link
status before routing messages and prompt users to pair via the web
interface. WebChannel handles /link <channel-type> <code> commands to
complete the pairing flow.

Fix inverted expiration check in IsPairingCodeExpired that caused codes
to be treated as expired immediately after creation.

Luna.Identity.Abstractions/PairingCode.cs

@@ -0,0 +1,12 @@
+namespace Luna.Identity.Abstractions;
+
+/// <summary>
+/// A short-lived code used to link a new channel to an existing user.
+/// Generated on a trusted channel (e.g., web), confirmed from the channel being linked.
+/// </summary>
+public record PairingCode(
+    string Code,
+    string ChannelType,
+    string UserId,
+    DateTimeOffset CreatedAt,
+    TimeSpan TimeToLive);