Cross-row conflicts (409)

The conflict pass checks uniqueness for the whole batch before writing: duplicates inside the payload and clashes with existing rows are a 409, not a per-row query or a 500 at commit time. Rules come from two places:

1. The model, automatically. Non-pk unique=True fields, Meta.unique_together, and unconditional field-only UniqueConstraints become rules whenever all their fields are writable on the serializer. DRF's per-row UniqueValidator/UniqueTogetherValidator for those rules are removed, so validation does no per-row queries. Opt out with Meta.batch_model_unique = False.
2. Explicitly, for rules the model does not declare:

# quickstart_app/models.py: give Order a client-supplied reference
class Order(models.Model):
    ...
    reference_id = models.CharField(max_length=64)


# quickstart_app/serializers.py
class ConflictOrderSerializer(OrderSerializer):
    class Meta(OrderSerializer.Meta):
        fields = (*OrderSerializer.Meta.fields, "reference_id")
        # one rule per tuple entry: a single field name, or a composite
        # (tuple of field names). Every name must be a concrete field, an
        # FK, or the pk on Meta.model (ImproperlyConfigured otherwise).
        batch_unique_together = ("reference_id",)

curl -X POST http://localhost:8000/conflict-orders/batch/ \
  -H 'Content-Type: application/json' \
  -d '[
    {"ticker": "1101", "customer": "112002", "volume": 1, "reference_id": "DUP"},
    {"ticker": "1102", "customer": "112003", "volume": 2, "reference_id": "DUP"}
  ]'

[
  {"reference_id": ["Conflicts with another row in this batch."]},
  {"reference_id": ["Conflicts with another row in this batch."]}
]

• Two rows in the same payload with the same rule value are a 409 on every row of the group ("Conflicts with another row in this batch.").
• A row whose rule value matches an existing row (other than its own target, for updates) is a 409 ("Conflicts with an existing record.").
• NULL values never conflict, matching SQL UNIQUE.
• The check is one query per rule, against the whole table (uniqueness is not tenant-scoped).
• An update row that omits a rule field is checked against the field's current value, so a row keeping its own value never conflicts with itself.
• The status is overridable per view: batch_conflict_status = 422.