SEO & care instructions on product sync

POST /api/v1/seller-erp/products accepts SEO and care fields on the same create/update endpoint — no new URLs.

Seller-side configuration mapping:

• Care Instructions → care_instructions (or fabric_care), max 1000 — apparel wash/care text shown on the product page
• Short Description → short_description, max 500 — listing summary and SEO fallback
• Meta Title → meta_title, max 255
• Meta Description → meta_description, max 500
• Meta Keywords → meta_keywords, max 500 — comma-separated

Apparel payloads should also include available_sizes, colours, and size_chart. Download product_create and product_create_apparel from Sample Payloads or GET /api/v1/seller-erp/samples/payloads.

Live responses return care_instructions and fabric_care (same value) plus the SEO fields.

See docs/PRODUCT_ONBOARDING_API.md, docs/SELLER_ERP_API.md, and the in-portal Seller ERP Integration Guide.