feat: add publish-only NostrClient and NostrEvent model

Stripped-down Nostr client that connects to nostrclient's internal
WebSocket for publishing NIP-52 calendar events. No subscription
capabilities — publish queue only.

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

nostr/event.py

@@ -0,0 +1,27 @@
+import hashlib
+import json
+from typing import List, Optional
+
+from pydantic import BaseModel
+
+
+class NostrEvent(BaseModel):
+    id: str = ""
+    pubkey: str
+    created_at: int
+    kind: int
+    tags: List[List[str]] = []
+    content: str = ""
+    sig: Optional[str] = None
+
+    def serialize(self) -> List:
+        return [0, self.pubkey, self.created_at, self.kind, self.tags, self.content]
+
+    def serialize_json(self) -> str:
+        e = self.serialize()
+        return json.dumps(e, separators=(",", ":"), ensure_ascii=False)
+
+    @property
+    def event_id(self) -> str:
+        data = self.serialize_json()
+        return hashlib.sha256(data.encode()).hexdigest()