fix(api): add ep_id to EPUpdate schema so extension point IDs can be patched

EPUpdate was missing the ep_id field, making it impossible to assign a
human-readable ID to an existing EP via PATCH. The router already uses
model_dump(exclude_unset=True) + setattr so no router change needed.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

state-hub/api/schemas/extension_point.py

@@ -22,6 +22,7 @@ class EPCreate(BaseModel):
 
 
 class EPUpdate(BaseModel):
+    ep_id: str | None = None
     title: str | None = None
     description: str | None = None
     location: str | None = None