Implement offer CRUD, filtering, pagination, and image upload

[FriggemannMichael]

Objective:
Implement offer list, create, retrieve, update, delete, detail retrieval, filtering, search, ordering, pagination, and image upload behavior.

Acceptance criteria:

• GET /api/offers/ returns paginated count/results data.
• Supported query params include creator_id, search, ordering, page, max_delivery_time, min_price, and page_size where applicable.
• POST /api/offers/ requires a business user and exactly three details.
• PATCH /api/offers/{id}/ supports JSON detail updates and FormData image updates.
• DELETE /api/offers/{id}/ is restricted to the offer owner.
• GET /api/offerdetails/{id}/ returns the documented detail structure.

Frontend notes:
The frontend creates the offer first as JSON and uploads image afterwards via PATCH FormData.

[FriggemannMichael]

TDD workflow for this project:

• Define the expected behavior before implementing the endpoint or model behavior.
• Write or outline the failing test first.
• Implement only enough code to make that test pass.
• Refactor after the test passes.
• Keep Django check, Ruff, and the relevant tests green from the local virtual environment.

[FriggemannMichael]

Completed on main. Offer CRUD, detail retrieval, filtering, search, ordering, pagination, permissions, image upload, and documented response fixes are implemented and covered by tests.