feat: add NIP-52 Nostr publish + sync of calendar events

Approved events are mirrored to Nostr as NIP-52 calendar events (kind
31922) signed by the wallet owner's pubkey, and incoming kind 31922/31923
events from subscribed relays are synced into the local DB so events
created on other LNbits instances or Nostr clients show up locally.

- m009 stores nostr_event_id + nostr_event_created_at on each event
  (used for replaceable updates and NIP-09 deletes); m011 adds location
  + JSON-encoded categories list (NIP-52 location/`t` tags).
- models: Event/PublicEvent/CreateEvent gain location, categories,
  nostr_event_id, nostr_event_created_at; parse_categories validator
  decodes the JSON column on read.
- nostr/{event,nostr_client}.py: Schnorr signing, websocket relay client,
  and a NostrEvent model (publish-only and subscribe variants).
- nostr_publisher.py: build/sign NIP-52 kind 31922 events and NIP-09
  delete events; publish via the relay client.
- nostr_sync.py: subscribe to kinds 31922/31923, dedupe by nostr_event_id
  / d-tag, upsert Events; auto-approves discovered Nostr events since
  they're already public.
- nostr_hooks.py: thin bridge that views_api handlers call to publish
  or delete a NIP-52 event for a given local event. Lives in its own
  module to keep `from . import nostr_client` out of the view layer
  and avoid the views_api -> publisher import cycle.
- views_api: hooks publish_or_delete_nostr_event into create-on-approved,
  update-when-already-published, cancel (delete), delete (delete), and
  approve (publish).
- __init__.py: 3-task lifespan — wait_for_paid_invoices (upstream),
  NostrClient bootstrap, and the NIP-52 sync loop. Module-level
  nostr_client global is set by the bootstrap and read dynamically by
  publish_or_delete_nostr_event so the import order works regardless of
  whether nostrclient is up at startup.

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

nostr_publisher.py

@@ -0,0 +1,118 @@
+"""
+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
+
+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])
+    if event.location:
+        tags.append(["location", event.location])
+    for cat in (event.categories or []):
+        tags.append(["t", cat])
+
+    nostr_event = NostrEvent(
+        pubkey=pubkey,
+        created_at=int(time.time()),
+        kind=31922,
+        tags=tags,
+        content=event.info or "",
+    )
+    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,
+) -> NostrEvent | None:
+    """
+    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