Add atomic batch creation for stored schedules

[PhiLily]

Expose django_ox.stored.create_schedules(rows, *, user=None) as a public,
stable API for schedules that should exist together.

The function validates every row and requires unique names within the
batch. One ValidationError reports every validation failure by row
index, name, field and message. Permission checks follow validation,
with PermissionDenied naming every denied row before any write.

The batch shares one clock reading. A write-only transaction on the
destination database saves every schedule and tells workers once.
A concurrent name conflict raises IntegrityError and rolls back the
whole batch. An empty batch returns [].

What changes for callers

• Use create_schedules for all-or-nothing batch creation. Pass a list of
  mappings with the fields create_schedule accepts.
• Read each validation entry's params for index, name, field and
  message. The index is zero-based; field is "" for a failure that
  belongs to no one field.
• With schedules routed to a database with a narrower integer range,
  create_schedule, update_schedule and the admin form now report field
  errors for values that would previously fail at the database write.
• Handle ValidationError for non-numeric values in every_seconds,
  phase_seconds or starting_deadline_seconds, phase_seconds=None, mixed
  naive and aware time bounds, and empty values in every_seconds,
  starting_deadline_seconds or end_time. These inputs previously raised
  TypeError in create_schedule and at the validation step in
  update_schedule. In update_schedule, non-numeric every_seconds and
  phase_seconds still raise a ValidationError with no field at an
  earlier step; that behavior is unchanged.
• On SQLite, create_schedules shares create_schedule's existing limit:
  both fail immediately with "database is locked" inside a caller's
  transaction.atomic() that has already read if another connection
  holds the write lock.

The stored-schedule write functions are documented as public. No
migration is needed. Stored-schedule dispatch is unchanged.

Tests cover each behavior the change adds or alters.