Implement auto-created M2M through table create/delete

[vgvoleg]

10. Implement auto-created M2M through table create/delete

Suggested milestone: M3 Django Contrib Smoke

Suggested labels: priority:p0, area:django-contrib, area:schema,
type:enhancement, status:confirmed-gap

Context

Django's base schema editor creates auto-created M2M through tables during
create_model(). The current backend override creates the model table and
indexes, but does not mirror that auto-created through table behavior.

Work

• Create auto-created M2M through tables after the primary model table.
• Delete auto-created M2M through tables when deleting models where appropriate.
• Type through table *_id columns from target primary key fields.
• Ensure no FK, REFERENCES, or ON DELETE SQL is emitted.
• Test auto-created through models and custom through models separately.
• Test target PK types: integer, string, UUID, and big integer.

Acceptance Criteria

• Auto-created through tables exist after create_model().
• M2M add/list/remove works at the ORM level.
• Relation docs match tested behavior.

Dependencies

Blocked by:

• Make unsupported constraints and schema operations explicit

Unblocks:

• Add Django contrib smoke test project

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