✨(backend) add list of seats of batch order API client (v2 sales tunnel)

[jonathanreveille]

Purpose

We want to show the list of the learners that will take part in the batch order. We need to list their fullnames and the voucher code that are available in the batch order.

Proposal

• Add nested batch order seats viewset
• Add filterset by owner name
• Add addition indicators to BatchOrderSerializer API client (seats_owned & seats_to_own)

[Copilot]

Pull request overview

This PR adds client-facing support for exposing batch order “seat” information: aggregated seat counters on batch orders, plus a nested endpoint to list/retrieve the individual seats (orders) and their learner/voucher info.

Changes:

• Added seats_owned / seats_to_own indicators on BatchOrder and exposed them via client serializers (batch orders, quotes, agreements).
• Introduced a nested /batch-orders/{batch_order_id}/seats/ API (serializer + filter + routing) to list/retrieve batch-order seats.
• Updated OpenAPI snapshot and expanded test coverage (including perf baselines) for the new fields and endpoint.

Reviewed changes

Copilot reviewed 16 out of 16 changed files in this pull request and generated 7 comments.

Show a summary per file

File	Description
src/backend/joanie/tests/swagger/swagger.json	Adds schema/paths for nested batch-order seats + new seat counters in batch-order schemas.
src/backend/joanie/tests/core/models/test_batch_order.py	Adds unit tests for BatchOrder.seats_owned and BatchOrder.seats_to_own.
src/backend/joanie/tests/core/api/quotes/test_api_quotes.py	Updates expected quote payload to include seat counters under batch_order.
src/backend/joanie/tests/core/api/organizations/test_api_organizations_quotes.py	Updates organization quote API expectations to include seat counters.
src/backend/joanie/tests/core/api/organizations/test_api_organizations_agreements.py	Updates agreement API expectations to include seat counters.
src/backend/joanie/tests/core/api/organizations/test_api_organizations_agreements.perf.yml	Updates perf baseline to account for extra queries from new counters.
src/backend/joanie/tests/core/api/batch_order/test_retrieve.py	Updates batch order retrieve expectations to include seat counters.
src/backend/joanie/tests/core/api/batch_order/test_retrieve.perf.yml	Updates perf baseline for batch order retrieve.
src/backend/joanie/tests/core/api/batch_order/test_nested_batch_order_seats.py	New test suite covering nested seats list/retrieve, permissions, pagination, and filtering.
src/backend/joanie/tests/core/api/batch_order/test_list.py	Updates batch order list expectations to include seat counters.
src/backend/joanie/tests/core/api/batch_order/test_list.perf.yml	Updates perf baseline for batch order list.
src/backend/joanie/core/serializers/client.py	Exposes seat counters and adds NestedBatchOrderSeatsSerializer.
src/backend/joanie/core/models/products.py	Adds seats_owned / seats_to_own properties to BatchOrder.
src/backend/joanie/core/filters/client/init.py	Adds a filterset for nested batch-order seats (query filter).
src/backend/joanie/core/api/client/init.py	Introduces GenericBatchOrderViewSet and adds NestedBatchOrderSeatsViewSet.
src/backend/joanie/client_urls.py	Registers nested batch-order seats routes under /batch-orders/{batch_order_id}/.

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