fix: queue outgoing events when relay connection is down

When all relay connections are temporarily lost, EVENT messages published
by extensions (nostrmarket, events) are now queued in a bounded deque
(max 100) instead of being silently dropped. On reconnection, queued
events are flushed to all connected relays. Dead relay queues are also
drained before restart to preserve in-flight events.

Closes aiolabs/nostrclient#1

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

nostr/relay.py

@@ -1,6 +1,9 @@
+from __future__ import annotations
+
 import asyncio
 import json
 import time
+from collections.abc import Callable
 from queue import Queue
 
 from loguru import logger
@@ -28,6 +31,7 @@ class Relay:
         self.num_subscriptions: int = 0
 
         self.queue: Queue = Queue()
+        self.on_connect: Callable[[Relay], None] | None = None
 
     def connect(self):
         self.ws = WebSocketApp(
@@ -97,6 +101,11 @@ class Relay:
         logger.info(f"[Relay: {self.url}] Connected.")
         self.connected = True
         self.shutdown = False
+        if self.on_connect:
+            try:
+                self.on_connect(self)
+            except Exception as e:
+                logger.warning(f"[Relay: {self.url}] on_connect callback error: {e}")
 
     def _on_close(self, _, status_code, message):
         logger.warning(