feat: add NIP-52 calendar event builder and publisher

- build_nip52_event(): Event model → kind 31922 NostrEvent with
  d, title, start, end, image tags
- build_nip52_delete_event(): kind 5 delete with 'a' tag per NIP-09
- sign_nostr_event(): Schnorr signing via coincurve
- publish_event_to_nostr(): build + sign + publish, returns event
  for metadata storage. Graceful failure (returns None).

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

nostr_publisher.py

@@ -0,0 +1,115 @@
+"""
+NIP-52 calendar event publishing for the events extension.
+
+Builds kind 31922 (date-based) calendar events from the Event model,
+signs them with the event creator's Account keypair, and publishes
+via the NostrClient to nostrclient relays.
+
+Reference: https://github.com/nostr-protocol/nips/blob/master/52.md
+"""
+
+import time
+from typing import Optional
+
+import coincurve
+from loguru import logger
+
+from .models import Event
+from .nostr.event import NostrEvent
+
+
+def build_nip52_event(event: Event, pubkey: str) -> NostrEvent:
+    """
+    Convert an Event model to a NIP-52 kind 31922 (date-based) calendar event.
+
+    Tags:
+      d       - event.id (addressable identifier)
+      title   - event.name
+      start   - event.event_start_date (ISO date string)
+      end     - event.event_end_date (optional)
+      image   - event.banner (optional)
+    Content:  event.info (description)
+    """
+    tags = [
+        ["d", event.id],
+        ["title", event.name],
+        ["start", event.event_start_date],
+    ]
+
+    if event.event_end_date:
+        tags.append(["end", event.event_end_date])
+    if event.banner:
+        tags.append(["image", event.banner])
+
+    nostr_event = NostrEvent(
+        pubkey=pubkey,
+        created_at=int(time.time()),
+        kind=31922,
+        tags=tags,
+        content=event.info,
+    )
+    nostr_event.id = nostr_event.event_id
+    return nostr_event
+
+
+def build_nip52_delete_event(event: Event, pubkey: str) -> NostrEvent:
+    """
+    Build a kind 5 delete event for a published NIP-52 calendar event.
+
+    Uses an 'a' tag to reference the parameterized replaceable event
+    (kind 31922) per NIP-09.
+    """
+    nostr_event = NostrEvent(
+        pubkey=pubkey,
+        created_at=int(time.time()),
+        kind=5,
+        tags=[
+            ["a", f"31922:{pubkey}:{event.id}"],
+        ],
+        content="Event canceled",
+    )
+    nostr_event.id = nostr_event.event_id
+    return nostr_event
+
+
+def sign_nostr_event(nostr_event: NostrEvent, private_key_hex: str) -> None:
+    """Sign a NostrEvent in-place using Schnorr signature."""
+    privkey = coincurve.PrivateKey(bytes.fromhex(private_key_hex))
+    sig = privkey.sign_schnorr(bytes.fromhex(nostr_event.id))
+    nostr_event.sig = sig.hex()
+
+
+async def publish_event_to_nostr(
+    nostr_client,
+    event: Event,
+    account_pubkey: str,
+    account_prvkey: str,
+    delete: bool = False,
+) -> Optional[NostrEvent]:
+    """
+    Build, sign, and publish a NIP-52 calendar event (or delete event).
+
+    Returns the published NostrEvent for metadata storage, or None on failure.
+    """
+    if not nostr_client:
+        logger.debug("[EVENTS] No NostrClient available, skipping publish")
+        return None
+
+    try:
+        if delete:
+            nostr_event = build_nip52_delete_event(event, account_pubkey)
+        else:
+            nostr_event = build_nip52_event(event, account_pubkey)
+
+        sign_nostr_event(nostr_event, account_prvkey)
+        await nostr_client.publish_nostr_event(nostr_event)
+
+        logger.info(
+            f"[EVENTS] Published NIP-52 {'delete' if delete else 'calendar'} "
+            f"event: {nostr_event.id[:16]}... (kind {nostr_event.kind})"
+        )
+        return nostr_event
+
+    except Exception as e:
+        logger.warning(f"[EVENTS] Failed to publish to Nostr: {e}")
+        return None