test: add platform API integration tests (67 tests)

Add comprehensive test coverage for platform marketing homepage API:
- test_pricing.py: 17 tests for tiers, add-ons, pricing endpoints
- test_letzshop_vendors.py: 22 tests for vendor lookup and claiming
- test_signup.py: 28 tests for multi-step signup flow

Fix signup.py to use correct password hashing from middleware/auth.py
and properly create Company with owner_user_id.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

app/api/v1/platform/signup.py

@@ -267,30 +267,43 @@ async def create_account(
         )
 
     try:
-        # Create Company
+        # Create User first (needed for Company owner)
+        from middleware.auth import AuthManager
+
+        auth_manager = AuthManager()
+
+        # Generate username from email
+        username = request.email.split("@")[0]
+        base_username = username
+        counter = 1
+        while db.query(User).filter(User.username == username).first():
+            username = f"{base_username}_{counter}"
+            counter += 1
+
+        user = User(
+            email=request.email,
+            username=username,
+            hashed_password=auth_manager.hash_password(request.password),
+            first_name=request.first_name,
+            last_name=request.last_name,
+            role="vendor",
+            is_active=True,
+        )
+        db.add(user)
+        db.flush()
+
+        # Create Company (with owner)
         from models.database.company import Company
 
         company = Company(
             name=request.company_name,
+            owner_user_id=user.id,
             contact_email=request.email,
             contact_phone=request.phone,
         )
         db.add(company)
         db.flush()
 
-        # Create User
-        from app.core.security import get_password_hash
-
-        user = User(
-            email=request.email,
-            hashed_password=get_password_hash(request.password),
-            first_name=request.first_name,
-            last_name=request.last_name,
-            is_active=True,
-        )
-        db.add(user)
-        db.flush()
-
         # Generate vendor code
         vendor_code = request.company_name.upper().replace(" ", "_")[:20]
         # Ensure unique