v1.0.1 - Review Fixes

Fixes from second code review

• Add editable=False to db_default=Now() in correct-sample fixture (prevents field showing in forms/admin)
• Add version note for CheckConstraint(condition=...) kwarg (Django 5.1+ syntax; use check= on 5.0)
• Replace deprecated unique_together with UniqueConstraint in through-model example (was contradicting plugin's own teaching)
• Fix USE_TZ version attribution: runtime default changed in Django 4.0, not 5.0
• Fix PositiveIntegerField description: allows values 0-2147483647 (not ">= 0")
• Add read_only_fields to PostCreateSerializer fixture (was contradicting plugin's own DRF rules)