fix: make wallet optional in CreateEvent for propose endpoint

The propose endpoint sets wallet from the authenticated user's
invoice key. Making wallet optional in the model allows the
request body to omit it. The admin create endpoint falls back
to the auth wallet if not provided.

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

models.py

@@ -28,7 +28,7 @@ class EventExtra(BaseModel):
 
 
 class CreateEvent(BaseModel):
-    wallet: str
+    wallet: Optional[str] = None
     name: str
     info: str
     closing_date: str