✨(backend) add --owned-batch-orders option to create_dev_data

[kernicPanel]

Purpose

When running make dev-data, batch orders are created without an explicit
owner, which makes it impossible to exercise the "owned batch orders" flow
as the user connected on Richie (user10). The code to wire user10 onto
batch orders existed but was commented out, so there was no way to opt in.

On top of that, make dev-data required the awkward
ARGS="--credential" form to forward flags to create_dev_data, which
hurt discoverability when stacking multiple flags.

Proposal

• Uncomment the user10 batch-order owner fixture in generate_simple
  and gate it behind a new owned_batch_orders flag (disabled by
  default). The flag also surfaces in the interactive select_multiple
  menu as "Owned batch orders".
• Expose the flag as --owned-batch-orders on the
  create_dev_data management command.
• Rework the dev-data Makefile target so flags after -- are
  forwarded to create_dev_data, enabling:
  make dev-data -- --owned-batch-orders --credential

[Copilot]

Pull request overview

This PR improves the dev dataset generation workflow by making it possible to generate batch orders explicitly owned by Richie’s default user (user10), and by making it easier to forward flags to the create_dev_data Django management command from make dev-data.

Changes:

• Add an owned_batch_orders toggle to Demo.generate_simple, including an interactive menu entry.
• Expose the toggle as --owned-batch-orders on the create_dev_data management command.
• Rework the dev-data Makefile target to forward extra CLI tokens to create_dev_data.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 3 comments.

File	Description
src/backend/joanie/tests/testing_utils.py	Adds an owned_batch_orders flag and gates creation/assignment of the user10 batch-order owner and related order ownership updates.
src/backend/joanie/demo/management/commands/create_dev_data.py	Adds the --owned-batch-orders CLI flag and forwards it into Demo.generate_simple.
Makefile	Introduces argument forwarding from make dev-data … to manage.py create_dev_data.

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