Add supplier product/price association implementation plan

Document comprehensive plan for implementing critical missing feature:
- Supplier-product associations with pricing
- Required for automatic Purchase Order (PO) creation
- Backend SupplierPriceList model already exists
- Frontend implementation needed

Plan includes:
- UI/UX design for product selection
- Price entry forms
- API hooks needed
- Data flow diagrams
- Implementation checklist

This addresses the critical gap preventing automatic PO functionality.

SUPPLIER_PRODUCT_ASSOCIATION_PLAN.md

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+# Supplier Product/Price Association Implementation Plan
+
+## Critical Feature for Automatic PO Creation
+
+### Problem
+Suppliers currently have no product/price associations. Without this data:
+- ❌ Automatic Purchase Order (PO) creation cannot function
+- ❌ System cannot determine which supplier to order from
+- ❌ System cannot calculate PO amounts
+- ❌ Procurement optimization is impossible
+
+### Backend Support (Already Exists)
+`SupplierPriceList` model in `/services/suppliers/app/models/suppliers.py`:
+```python
+class SupplierPriceList(Base):
+    supplier_id: UUID
+    inventory_product_id: UUID  # Reference to product
+    unit_price: Decimal         # Price per unit
+    unit_of_measure: String     # kg, g, L, ml, units
+    minimum_order_quantity: Integer
+    tier_pricing: JSONB         # Volume discounts
+    effective_date: DateTime
+    expiry_date: DateTime
+    is_active: Boolean
+```
+
+### Frontend Implementation
+
+#### Phase 1: Supplier Step Enhancement
+**File**: `/frontend/src/components/domain/setup-wizard/steps/SuppliersSetupStep.tsx`
+
+**New Features**:
+1. **Product Association Section** (after supplier is created)
+   - Expandable "Manage Products" for each supplier
+   - Multi-select product picker from inventory
+   - Price entry form for each selected product
+
+2. **UI Flow**:
+```
+[Supplier Card]
+├─ Name, Contact, Type
+├─ [Manage Products ▼] button
+└─ When expanded:
+    ├─ [+ Add Product] button → Opens modal
+    ├─ Product List (if any exist):
+    │   └─ [Product Name] - [Price] [Unit] [Edit] [Delete]
+    └─ [Save Products] button
+```
+
+3. **Product Selector Modal**:
+```
+Modal: "Add Products for [Supplier Name]"
+├─ Multi-select dropdown (from inventory)
+├─ For each selected product:
+│   ├─ Product Name (read-only)
+│   ├─ Unit Price (€) input *required
+│   ├─ Unit of Measure select *required
+│   └─ Min Order Qty input (optional)
+└─ [Cancel] [Save Prices]
+```
+
+#### Phase 2: Backend Integration
+**API Endpoints** (already exist):
+- `POST /suppliers/{supplier_id}/price-lists` - Create price list item
+- `GET /suppliers/{supplier_id}/price-lists` - Get supplier's price list
+- `PUT /suppliers/{supplier_id}/price-lists/{price_list_id}` - Update price
+- `DELETE /suppliers/{supplier_id}/price-lists/{price_list_id}` - Delete price
+
+**Frontend Services** needed:
+- `useSupplierPriceLists(supplierId)` - Fetch price lists
+- `useCreateSupplierPriceList()` - Create price list items
+- `useUpdateSupplierPriceList()` - Update prices
+- `useDeleteSupplierPriceList()` - Delete price list items
+
+#### Phase 3: Data Flow
+```
+User creates supplier
+    ↓
+Supplier card shows "Manage Products" button
+    ↓
+Click → Expand section showing current products (if any)
+    ↓
+Click "Add Product" → Modal opens
+    ↓
+Select products from inventory + Enter prices
+    ↓
+Save → API call to create SupplierPriceList entries
+    ↓
+Modal closes, product list updates
+    ↓
+User can proceed to next step
+```
+
+### Implementation Checklist
+
+- [ ] Create useSupplierPriceLists hook
+- [ ] Create useCreateSupplierPriceList hook
+- [ ] Create useUpdateSupplierPriceList hook
+- [ ] Create useDeleteSupplierPriceList hook
+- [ ] Add product management UI to SuppliersSetupStep
+- [ ] Create product selector modal component
+- [ ] Add price entry form
+- [ ] Integrate with backend API
+- [ ] Add validation (price > 0, unit required)
+- [ ] Test create/update/delete flow
+- [ ] Test navigation (can proceed after products added)
+
+### Benefits After Implementation
+✅ Automatic PO creation will work
+✅ System knows which supplier supplies what
+✅ System knows current prices
+✅ Can calculate PO totals automatically
+✅ Can optimize procurement based on prices
+✅ Can track price changes over time
+✅ Foundation for supplier performance analysis