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/schemas/hosted_site.py

@@ -3,18 +3,31 @@
 
 from datetime import datetime
 
-from pydantic import BaseModel, Field
+from pydantic import BaseModel, Field, model_validator
 
 
 class HostedSiteCreate(BaseModel):
-    """Schema for creating a hosted site."""
+    """Schema for creating a hosted site.
+
+    Either merchant_id or prospect_id must be provided:
+    - merchant_id: store is created under this merchant
+    - prospect_id: a merchant is auto-created from prospect data
+    """
 
     business_name: str = Field(..., max_length=255)
+    merchant_id: int | None = None
+    prospect_id: int | None = None
     contact_name: str | None = Field(None, max_length=255)
     contact_email: str | None = Field(None, max_length=255)
     contact_phone: str | None = Field(None, max_length=50)
     internal_notes: str | None = None
 
+    @model_validator(mode="after")
+    def require_merchant_or_prospect(self) -> "HostedSiteCreate":
+        if not self.merchant_id and not self.prospect_id:
+            raise ValueError("Either merchant_id or prospect_id is required")
+        return self
+
 
 class HostedSiteUpdate(BaseModel):
     """Schema for updating a hosted site."""