fix: Allow for later adding of django CMS versioning

[fsbraun]

Description

This PR removes a conditional dependency from the migration files and allows to add versioning to snippets after having executed all migrations.

This is particular important when upgrading from django CMS 3 ( no versioning) to django CMS 4+ (potentially with versioning).

The settings are added to the README.

Related resources

• [BUG] Adding djangocms-versioning after updating to djangocms-snippet 5 breaks migrations #183
• #...

Checklist

• I have opened this pull request against master
• I have added or modified the tests when changing logic
• I have followed the conventional commits guidelines to add meaningful information into the changelog
• I have read the contribution guidelines and I have joined #pr-review on
  Discord to find a “pr review buddy” who is
  going to review my pull request.

Summary by Sourcery

Simplify the existing migration to remove djangocms_versioning dependencies so migrations can execute without versioning installed and allow versioning to be added post-migration.

Enhancements:

• Remove conditional imports, settings checks, content type creation, and version creation from the 0010 migration.
• Always assign a SnippetGrouper to each snippet regardless of versioning availability.
• Drop migration dependencies on djangocms_versioning to decouple snippet migrations from versioning.

Documentation:

• Document versioning-related settings and usage in the README.

[sourcery-ai]

Reviewer's Guide

This PR refactors the 0010 data migration to strip out all built-in versioning checks and dependencies—deferring version setup to run after migrations—and adds README entries for the new versioning feature flags.

File-Level Changes

Change	Details	Files
Refactor data migration to remove built-in versioning logic and external imports	Dropped imports and use of global_apps and create_contenttypes Removed settings-based feature flag and djangocms_versioning import/installation checks Eliminated Version creation block and migration user lookup Added db_alias and used .using(db_alias) on queryset Omitted versioning dependency in Migration.dependencies	src/djangocms_snippet/migrations/0010_cms4_grouper_version_data_migration.py
Document new versioning settings in README	Added DJANGOCMS_SNIPPET_VERSIONING_ENABLED description Added DJANGOCMS_SNIPPET_VERSIONING_MIGRATION_USER_ID description Updated README.rst with usage guidance	README.rst

Possibly linked issues

• [BUG] Adding djangocms-versioning after updating to djangocms-snippet 5 breaks migrations #183: PR modifies migration 0010, removing a dependency on versioning 0015 to fix the InconsistentMigrationHistory error.

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[sourcery-ai]

Hey @fsbraun - I've reviewed your changes - here's some feedback:

• Specify a reverse_code (or use migrations.RunPython.noop) for the cms4_grouper_version_migration to make the migration reversible.
• Use snippet.save(using=db_alias) when persisting to ensure writes go to the same database alias used for reads.
• Add an explicit dependency on the contenttypes app or re-introduce create_contenttypes to guarantee ContentType entries exist before this migration runs.

Here's what I looked at during the review

• 🟡 General issues: 3 issues found
• 🟢 Security: all looks good
• 🟢 Testing: all looks good
• 🟢 Complexity: all looks good
• 🟢 Documentation: all looks good

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