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>

models.py

@@ -1,3 +1,4 @@
+import json
 from datetime import datetime
 
 from pydantic import BaseModel, EmailStr, Field, root_validator, validator
@@ -42,6 +43,8 @@ class CreateEvent(BaseModel):
     amount_tickets: int = 0  # 0 = unlimited / not ticketed
     price_per_ticket: float = 0  # 0 = free
     banner: str | None = None
+    location: str | None = None  # venue/address (NIP-52 'location' tag)
+    categories: list[str] = Field(default_factory=list)  # NIP-52 't' tags
     extra: EventExtra = Field(default_factory=EventExtra)
     status: str = "approved"  # proposed, approved, rejected
 
@@ -63,8 +66,18 @@ class Event(BaseModel):
     time: datetime
     sold: int = 0
     banner: str | None = None
+    location: str | None = None
+    categories: list[str] = Field(default_factory=list)
     extra: EventExtra = Field(default_factory=EventExtra)
     status: str = "approved"
+    nostr_event_id: str | None = None
+    nostr_event_created_at: int | None = None
+
+    @validator("categories", pre=True)
+    def parse_categories(cls, v):
+        if isinstance(v, str):
+            return json.loads(v) if v else []
+        return v or []
 
 
 class PublicEvent(BaseModel):
@@ -80,9 +93,17 @@ class PublicEvent(BaseModel):
     fiat_currency: str = "GBP"
     price_per_ticket: float
     banner: str | None
+    location: str | None = None
+    categories: list[str] = Field(default_factory=list)
     extra: EventExtra = Field(default_factory=EventExtra)
     status: str = "approved"  # surfaces "proposed"/"rejected" so SFC can render banner
 
+    @validator("categories", pre=True)
+    def parse_categories(cls, v):
+        if isinstance(v, str):
+            return json.loads(v) if v else []
+        return v or []
+
 
 class EventsSettings(BaseModel):
     """Extension-level settings for the events extension."""