fix(hosting): require merchant or prospect for site creation

- Schema: add merchant_id/prospect_id with model_validator requiring
  at least one. Remove from-prospect endpoint (unified into POST /sites)
- Service: rewrite create() — if merchant_id use it directly, if
  prospect_id auto-create merchant from prospect data. Remove system
  merchant hack entirely. Extract _create_merchant_from_prospect helper.
- Simplify accept_proposal() — merchant already exists at creation,
  only creates subscription and marks prospect converted
- Tests: update all create calls with merchant_id, replace from-prospect
  tests with prospect_id + validation tests

Closes docs/proposals/hosting-site-creation-fix.md

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

app/modules/hosting/routes/api/admin_sites.py

@@ -96,17 +96,6 @@ def create_site(
     return _to_response(site)
 
 
-@router.post("/from-prospect/{prospect_id}", response_model=HostedSiteResponse)
-def create_from_prospect(
-    prospect_id: int = Path(...),
-    db: Session = Depends(get_db),
-    current_admin: UserContext = Depends(get_current_admin_api),
-):
-    """Create a hosted site pre-filled from prospect data."""
-    site = hosted_site_service.create_from_prospect(db, prospect_id)
-    db.commit()
-    return _to_response(site)
-
 
 @router.put("/{site_id}", response_model=HostedSiteResponse)
 def update_site(