Modelling limited availability in Doctrine

This question targets modelling limited availability in Doctrine 2. I'm sure this has already been discussed here as it seems quite basic but I could not find any best practices. May it be that limit/restrict/max/... are bad search terms as they all mean something else in the db world :-).

Simplified example

Assume a typical online shop application that allows multiple users to buy items of some kind (at the same time). Some of these items may have a limited availability (first come first served). So two users may be in a concurrent situtation when trying to checkout/confirming the order. The faster one must win the race, the other order should not even be processed (inserted in the database).

Entities/tables may look like this:

items

+----+-----+---------------+---------+
| id | ... | max_available | version |
+----+-----|---------------|---------+
| 7  |     | 4             | 2       |
| 8  |     | 1             | 0       |

orders

+----+---------+----------+
| id | item_id | quantity |
+----+---------+----------+
| 1  | 7       | 2        |
| 1  | 7       | 1        |

In this case: Another order for item 8 with a quantity of 1 would be valid. Another order for item 7 with a quantity of 2 must be prevented as this would be one more that available.

Best practice?

The application uses Doctrine 2 ORM, the db will be MySQL. The system may be coupled to the db type but if there is a reasonable db agnostic way that's even better of course.

What's the best way to model this?

Transactions and locking on db level (db needs to support this)? Locking on ORM level (integer version field)? Or should there be (additionally) installed triggers that ensure data integrity on database level?

Sidenote: Should constraints be optional by design or can they be part of the business logic? In other words: Is it bad practice to test against constraints and let the test fail under normal conditions - e.g. by having a (concurrency safe) trigger on updates/inserts, that cancels the request if an item isn't available anymore? (This would only work for certain db types and InnoDB as the engine in the case of MySQL...)