add Nostr publishing and bidirectional sync

- nostr/ vendors NostrEvent + the nostrclient WebSocket bridge from
  the events extension, retagged [TASKS] / subscription-id "tasks-*".
- nostr_publisher builds kind 31922 with the `event-type: task` tag
  (per aiolabs/webapp#25 — disambiguates from kind-31922 activities on
  shared relays), kind 31925 with task-status / occurrence /
  completed_at, and kind 5 deletions for both.
- nostr_hooks bridges task/completion mutations to the publisher and
  persists the resulting nostr_event_id back onto the local row.
- nostr_sync subscribes to {31922, 31925, 5/#k} and filters 31922
  client-side on `event-type: task` because most relays don't index
  custom single-letter tags.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

nostr/event.py

@@ -0,0 +1,26 @@
+import hashlib
+import json
+
+from pydantic import BaseModel
+
+
+class NostrEvent(BaseModel):
+    id: str = ""
+    pubkey: str
+    created_at: int
+    kind: int
+    tags: list[list[str]] = []
+    content: str = ""
+    sig: str | None = 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()