Fix REQ message handling to support multiple filter subscriptions (#34)

This fix addresses an issue where REQ messages with multiple filters
were being rejected by the relay. Notably: The nostrmarket extension's
"Refresh from Nostr" functionality sends a single REQ message containing
4 different filter subscriptions:
- Direct Messages (kinds: [4])
- Stalls (kinds: [30017])
- Products (kinds: [30018])
- Profile (kinds: [0])

Changes:
- Changed validation from `len(data) != 3` to `len(data) < 3` to allow
multiple filters
- Added loop to process all filters in a single REQ message (data[2:])
- Accumulate responses from all filters before returning

This ensures compatibility with clients that batch multiple subscription
filters in a single REQ message, which is a valid pattern according to
NIP-01.

relay/client_connection.py

@@ -115,9 +115,15 @@ class NostrClientConnection:
             await self._handle_event(event)
             return []
         if message_type == NostrEventType.REQ:
-            if len(data) != 3:
+            if len(data) < 3:
                 return []
-            return await self._handle_request(data[1], NostrFilter.parse_obj(data[2]))
+            subscription_id = data[1]
+            # Handle multiple filters in REQ message
+            responses = []
+            for filter_data in data[2:]:
+                response = await self._handle_request(subscription_id, NostrFilter.parse_obj(filter_data))
+                responses.extend(response)
+            return responses
         if message_type == NostrEventType.CLOSE:
             self._handle_close(data[1])
         if message_type == NostrEventType.AUTH: