Offer Excel Import/Export Format (Seller)
=========================================

Overview
--------
- No ID columns are used. Import uses the authenticated seller only.
- Products and options are matched by `slug` (or `label`/`value` when noted).
- Additional items are matched by `additional_item_label` and must be approved.
- Export returns a ZIP of CSV files (each sheet = one CSV). Excel can edit CSVs.
- Import accepts the same ZIP of CSVs. Unknown columns are ignored.

Workbook Sheets (CSV Files)
---------------------------

File: Meta.csv
- key
- value

Suggested rows:
- template_version | 1
- generated_at | 2026-01-05T12:00:00Z

File: Offers.csv
- product_slug (required)
- unit_price (base price)
- stock
- currency
- status (enable/disable)

File: SellerTypes.csv
- product_slug (required)
- seller_type_slug (required)
- seller_type_item_slug (required)
- seller_type_group_name (optional; used if a type belongs to multiple groups)

File: StandardModifiers.csv
- product_slug (required)
- option_slug (required)
- option_item_slug (preferred) OR option_item_value
- operator (+ - % * /)
- value
- apply_on (base/current)

File: AdditionalModifiers.csv
- product_slug (required)
- additional_item_label (required)
- operator (+ - % * /)
- value
- apply_on (base/current)
- bound_option_slug (optional; helps disambiguate label)

File: LevelPricingRules.csv
- product_slug (required)
- option_slug (required, must be level type)
- mode (fixed/variant)
- min_level_label (required)
- max_level_label (required)
- operator (+ - % * /)
- per_step_value (required for fixed)
- apply_on (base/current)

File: LevelPricingSteps.csv
- product_slug (required)
- option_slug (required)
- from_level_label (required)
- to_level_label (required)
- operator (+ - % * /)
- value
- apply_on (base/current)

Rules
-----
- Rows with missing required fields are skipped with a row-level error.
- If the offer does not exist, `unit_price` is required to create it.
- Seller type selections are replaced only when valid rows exist for a product.
- Binding rules apply (seller type ↔ options/items); invalid rows fail with errors.
- Additional items must already exist, be approved, and belong to the seller.
- Level selections match by label (or numeric value if label is numeric).
- Large imports can be queued with `async=1`; poll the status endpoint for progress.

API Endpoints
-------------
- GET `/api/seller/offers/template` -> ZIP with CSV headers
- GET `/api/seller/offers/export` -> ZIP with CSV data
- POST `/api/seller/offers/import` -> accept ZIP, returns per-row errors (optional `async=1`)
- GET `/api/seller/offers/imports/{id}` -> poll async import status and row-level errors