Make 0001_initial idempotent and swappable-user-model safe (0.5.0)

[mpasternak]

Summary

Rework 0001_initial so it builds the dynamic_columns_* tables through the schema editor (create_model), guarded to run only when the tables are absent, instead of a plain CreateModel. The model side stays in state_operations, so Django's migration state is unchanged.

Two payoffs for downstream projects:

1. Pre-existing schema no longer collides. Projects extracted from an older in-tree app (notably BPP, whose dynamic_columns app created these tables years ago) can apply 0001_initial onto a database that already carries the tables — it detects them and no-ops. Such projects no longer need a MIGRATION_MODULES override; they can consume this migration directly.
2. Custom user models resolve correctly on a from-empty build. The user FK is materialised by the schema editor against settings.AUTH_USER_MODEL, so a fresh migrate / migrate --create-db on a project with a swapped user model points the FK at the right table instead of a hard-coded auth_user.

Why it's safe to change a released migration

• The end state is identical (same CreateModel pair + constraints, now under SeparateDatabaseAndState), so databases that already recorded 0001_initial as applied are untouched and makemigrations --check reports no drift.
• On a fresh install the guard's create_model produces exactly the same DDL as the old CreateModel.

Tests

• New tests/test_migrations.py: re-applies the initial migration onto pre-existing tables and asserts no collision (the legacy-upgrade scenario). Fails against the old plain CreateModel, passes now.
• Full suite green (55 passed) — every test relies on the fresh-build path for setup, so schema creation + constraints are exercised throughout.

Bumps version to 0.5.0 + CHANGELOG.

Downstream

BPP drops its bpp.migration_overrides.dynamic_admin_columns override and requires >=0.5.0 once this is released (iplweb/bpp PR forthcoming).

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