docs: add OMS positioning strategy and implementation plan

- Add "Lightweight OMS for Letzshop Sellers" positioning strategy
- Add back-office vs marketing positioning comparison
- Update pricing tiers for OMS model (Essential/Professional/Business)
- Add VAT invoice feature technical specification
- Add comprehensive OMS feature implementation plan
- Fix code block formatting in synology doc

New docs:
- docs/marketing/strategy/back-office-positioning.md
- docs/marketing/strategy/customer-marketing-positioning.md
- docs/implementation/vat-invoice-feature.md
- docs/implementation/oms-feature-plan.md

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

mkdocs.yml

@@ -147,6 +147,8 @@ nav:
       - Order Item Exceptions: implementation/order-item-exceptions.md
       - Product Suppliers Table: implementation/product-suppliers-table.md
       - Unified Order View: implementation/unified-order-view.md
+      - VAT Invoice Feature: implementation/vat-invoice-feature.md
+      - OMS Feature Plan: implementation/oms-feature-plan.md
     - Seed Scripts Audit: development/seed-scripts-audit.md
     - Database Seeder:
       - Documentation: development/database-seeder/database-seeder-documentation.md
@@ -222,6 +224,9 @@ nav:
     - Overview: marketing/index.md
     - Feature List & Roadmap: marketing/features.md
     - Pricing Strategy: marketing/pricing.md
+    - Strategy:
+      - Back-Office Positioning: marketing/strategy/back-office-positioning.md
+      - Customer & Marketing: marketing/strategy/customer-marketing-positioning.md
     - Outreach Templates:
       - Letzshop Automation: marketing/outreach/letzshop-automation.md
       - Website Vendors: marketing/outreach/website-vendors.md