refactor: move transaction management from services to API endpoints

- Services now use db.flush() instead of db.commit() for database operations
- API endpoints handle transaction commit after service calls
- Remove db.rollback() from services (let exception handlers manage this)
- Ensures consistent transaction boundaries at API layer

This pattern gives API endpoints full control over when to commit,
allowing for better error handling and potential multi-operation transactions.

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

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

app/services/customer_service.py

@@ -108,7 +108,7 @@ class CustomerService:
 
         try:
             db.add(customer)
-            db.commit()
+            db.flush()
             db.refresh(customer)
 
             logger.info(
@@ -120,7 +120,6 @@ class CustomerService:
             return customer
 
         except Exception as e:
-            db.rollback()
             logger.error(f"Error registering customer: {str(e)}")
             raise CustomerValidationException(
                 message="Failed to register customer", details={"error": str(e)}
@@ -310,7 +309,7 @@ class CustomerService:
                 setattr(customer, field, value)
 
         try:
-            db.commit()
+            db.flush()
             db.refresh(customer)
 
             logger.info(f"Customer updated: {customer.email} (ID: {customer.id})")
@@ -318,7 +317,6 @@ class CustomerService:
             return customer
 
         except Exception as e:
-            db.rollback()
             logger.error(f"Error updating customer: {str(e)}")
             raise CustomerValidationException(
                 message="Failed to update customer", details={"error": str(e)}
@@ -344,7 +342,7 @@ class CustomerService:
         customer = self.get_customer(db, vendor_id, customer_id)
         customer.is_active = False
 
-        db.commit()
+        db.flush()
         db.refresh(customer)
 
         logger.info(f"Customer deactivated: {customer.email} (ID: {customer.id})")
@@ -369,8 +367,6 @@ class CustomerService:
             customer.total_spent += order_total
             customer.last_order_date = datetime.utcnow()
 
-            db.commit()
-
             logger.debug(f"Updated stats for customer {customer.email}")
 
     def _generate_customer_number(