feat: add admin-toggleable auto-approve setting

- Extension settings table with auto_approve boolean
- GET/PUT /api/v1/settings endpoints (LNbits admin only)
- Settings card in admin UI with toggle
- When auto_approve is enabled, non-admin events skip approval

Closes aiolabs/events#11

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

models.py

@@ -84,6 +84,12 @@ class Event(BaseModel):
     nostr_event_created_at: int | None = None
 
 
+class EventsSettings(BaseModel):
+    """Extension-level settings for the events extension."""
+
+    auto_approve: bool = False  # Skip approval for all users
+
+
 class TicketExtra(BaseModel):
     applied_promo_code: str | None = None
     sats_paid: int | None = None