refactor(arch): move auth schemas to tenancy module and add cross-module service methods

Move all auth schemas (UserContext, UserLogin, LoginResponse, etc.) from
legacy models/schema/auth.py to app/modules/tenancy/schemas/auth.py per
MOD-019. Update 84 import sites across 14 modules. Legacy file now
re-exports for backwards compatibility.

Add missing tenancy service methods for cross-module consumers:
- merchant_service.get_merchant_by_owner_id()
- merchant_service.get_merchant_count_for_owner()
- admin_service.get_user_by_id() (public, was private-only)
- platform_service.get_active_store_count()

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

app/modules/tenancy/services/merchant_service.py

@@ -329,6 +329,49 @@ class MerchantService:
 
         return merchant, old_owner, new_owner
 
+    def get_merchant_by_owner_id(
+        self, db: Session, owner_user_id: int
+    ) -> Merchant | None:
+        """
+        Get merchant by owner user ID.
+
+        Args:
+            db: Database session
+            owner_user_id: Owner user ID
+
+        Returns:
+            First active merchant owned by the user, or None
+        """
+        return (
+            db.query(Merchant)
+            .filter(
+                Merchant.owner_user_id == owner_user_id,
+                Merchant.is_active == True,  # noqa: E712
+            )
+            .first()
+        )
+
+    def get_merchant_count_for_owner(
+        self, db: Session, owner_user_id: int, active_only: bool = True
+    ) -> int:
+        """
+        Count merchants owned by a user.
+
+        Args:
+            db: Database session
+            owner_user_id: Owner user ID
+            active_only: Only count active merchants
+
+        Returns:
+            Number of merchants
+        """
+        query = db.query(func.count(Merchant.id)).filter(
+            Merchant.owner_user_id == owner_user_id
+        )
+        if active_only:
+            query = query.filter(Merchant.is_active == True)  # noqa: E712
+        return query.scalar() or 0
+
     def get_merchant_stores(
         self, db: Session, merchant_id: int, skip: int = 0, limit: int = 100
     ) -> tuple[list, int]: