fix: preserve supported bounds in ox_import_beat_schedules

[ShivanshShukla]

• What and why:
  Preserves supported start_time and expires bounds when importing schedules from django-celery-beat via ox_import_beat_schedules, without performing database writes.

  • Introspects django_celery_beat_periodictask for one_off, start_time, and expires columns, falling back gracefully if absent on older table versions.
  • Emits future starts as start_time=datetime.fromisoformat(...) and future expiries as end_time=datetime.fromisoformat(...) using repr-based quoting, omitting past starts.
  • Includes from datetime import datetime in Section 2 imports output.
  • Lists one-off rows (one-off tasks have no equivalent on a stored schedule), expired rows (it has expired), and expiries at or before a future start (its expiry is at or before its start time) under # Not translated, and why:.
  • Interprets naive cursor values using connection.timezone when USE_TZ=True, and preserves naive local times when USE_TZ=False.
  • Corrects the footer's enabled-state claim, retains the fixed-instant explanation, and warns that an expiry near the present can pass before application, invalidating generated calls at creation.

• Tests: ran on SQLite and PostgreSQL (see CONTRIBUTING.md); new behavior is covered.

  • Updated make_beat_tables() fixture DDL using connection.data_types['DateTimeField'].
  • Updated Section 2 execution checks to include datetime in line-evaluation globals and verify preserved bounds.
  • Added test coverage for one-off task skips, expired task skips, expiry before future start skips, past start omission, future start/expiry bounds preservation, old table schemas missing columns, disabled task rows, and USE_TZ=True / USE_TZ=False timezone handling.

• Changelog: entry added under ## [Unreleased] ### Fixed in CHANGELOG.md.