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>

migrations.py

@@ -214,6 +214,19 @@ async def m008_add_event_status(db):
     )
 
 
+async def m009_add_nostr_columns(db):
+    """
+    Track the most recent NIP-52 calendar event we published for this event
+    (used for replaceable updates and NIP-09 deletes).
+    """
+    await _alter_add_column_safe(
+        db, "ALTER TABLE events.events ADD COLUMN nostr_event_id TEXT"
+    )
+    await _alter_add_column_safe(
+        db, "ALTER TABLE events.events ADD COLUMN nostr_event_created_at INTEGER"
+    )
+
+
 async def m010_add_events_settings(db):
     """
     Create the extension settings singleton row used by the admin UI to
@@ -232,3 +245,15 @@ async def m010_add_events_settings(db):
         "SELECT 1, FALSE WHERE NOT EXISTS "
         "(SELECT 1 FROM events.settings WHERE id = 1)"
     )
+
+
+async def m011_add_location_and_categories(db):
+    """
+    Add NIP-52 calendar metadata (location and a JSON-encoded category list).
+    """
+    await _alter_add_column_safe(
+        db, "ALTER TABLE events.events ADD COLUMN location TEXT"
+    )
+    await _alter_add_column_safe(
+        db, "ALTER TABLE events.events ADD COLUMN categories TEXT"
+    )