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/tests/conftest.py

@@ -89,6 +89,7 @@ def hosted_site(db, hosting_platform, system_merchant):
         db,
         {
             "business_name": f"Test Business {unique}",
+            "merchant_id": system_merchant.id,
             "contact_name": "John Doe",
             "contact_email": f"john-{unique}@example.com",
             "contact_phone": "+352 123 456",