Django/Python version upgrades

[snopoke]

Summary

• Drop Django 3.2 and Python 3.9 support (both EOL)
• Add Django 5.2 to the stable CI matrix
• Add Python 3.13 and 3.14 to the stable CI matrix
• Replace experimental Django 5.0.x job with django==6.0.* on Python 3.12
• Use CheckConstraint.condition on Django 5.1+ and check on older versions for cross-version compatibility

Test plan

• CI passes on Django 4.2 and 5.2 across Python 3.10–3.13
• Django 5.2 + Python 3.14 job passes
• Experimental Django 6.0 job runs (failures are non-blocking)
• Migration check step passes

🤖 Generated with Claude Code