fix: send only NIP-01 fields in EVENT responses

nostr_dict() returned every model field except relay_id and publisher,
so the size column added for storage accounting leaked into every
EVENT sent to clients. Emit the seven NIP-01 fields explicitly instead.
Caught by the client-flow test ported from upstream.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_013Tbyw6FwjhEJg3gHfPHxWt

relay/event.py

@@ -26,10 +26,17 @@ class NostrEvent(BaseModel):
     size: int = 0
 
     def nostr_dict(self) -> dict:
-        _nostr_dict = dict(self)
-        _nostr_dict.pop("relay_id")
-        _nostr_dict.pop("publisher")
-        return _nostr_dict
+        # Only the NIP-01 fields go on the wire; storage columns such as
+        # relay_id, publisher and size must not leak into EVENT responses.
+        return {
+            "id": self.id,
+            "pubkey": self.pubkey,
+            "created_at": self.created_at,
+            "kind": self.kind,
+            "tags": self.tags,
+            "content": self.content,
+            "sig": self.sig,
+        }
 
     def serialize(self) -> list:
         return [0, self.pubkey, self.created_at, self.kind, self.tags, self.content]