feat(issued-orders): counterparty is now a supplier (dodavatel), not a customer

Issued orders are purchase orders WE send - they must pick from suppliers
(sklad_suppliers), not from customers. Per user decision customer_id was
REPLACED (not kept alongside): migration drops issued_orders.customer_id and
adds supplier_id FK -> sklad_suppliers (existing rows lose their counterparty
- the feature is days old; re-point them in the UI).

- service: input/filters/search (suppliers.name + ico)/enrichment/detail all
  supplier-based; create validates the supplier inside the transaction and
  update before write -> Czech 400 'Dodavatel nenalezen' instead of P2003 500;
  detail returns a minimal supplier field set (no internal notes leak)
- routes: supplier_id on list + stats; new GET /issued-orders/suppliers
  lookup (orders.view/create/edit guard - orders users lack warehouse.manage
  which guards the warehouse suppliers CRUD), active suppliers only,
  name+id ordering
- PDF: Dodavatel block now renders the supplier (name, newline-split address,
  IC/DIC), layout and both language label sets unchanged
- frontend: new SupplierPicker kit component (CustomerPicker untouched),
  IssuedOrderDetail/IssuedOrders switched to supplier_id/supplier_name; the
  picker keeps a fallback option for orders whose supplier was later
  deactivated; WarehouseSuppliers CRUD now also invalidates issued-orders so
  the picker can't go stale
- tests: issued-orders suite switched to supplier fixtures + new coverage
  (lookup shape + 403, nonexistent supplier 400, PDF supplier block)

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

prisma/schema.prisma

@@ -193,7 +193,6 @@ model customers {
   sync_version  Int?         @default(0)
   invoices      invoices[]
   orders        orders[]
-  issued_orders issued_orders[]
   projects      projects[]
   quotations    quotations[]
 }
@@ -370,7 +369,7 @@ model orders {
 model issued_orders {
   id              Int                  @id @default(autoincrement())
   po_number       String?              @unique(map: "idx_issued_orders_number_unique") @db.VarChar(50)
-  customer_id     Int?
+  supplier_id     Int?
   status          issued_orders_status @default(draft)
   currency        String?              @default("CZK") @db.VarChar(10)
   vat_rate        Decimal?             @default(21.00) @db.Decimal(5, 2)
@@ -387,9 +386,9 @@ model issued_orders {
   created_at      DateTime?            @default(now()) @db.DateTime(0)
   modified_at     DateTime?            @db.DateTime(0)
   issued_order_items issued_order_items[]
-  customers       customers?           @relation(fields: [customer_id], references: [id], onDelete: Restrict, onUpdate: NoAction, map: "issued_orders_ibfk_1")
+  suppliers       sklad_suppliers?     @relation(fields: [supplier_id], references: [id], onDelete: Restrict, onUpdate: NoAction, map: "issued_orders_supplier_fk")
 
-  @@index([customer_id], map: "issued_orders_customer_id")
+  @@index([supplier_id], map: "issued_orders_supplier_id")
   @@index([status, order_date], map: "idx_issued_orders_status_date")
 }
 
@@ -796,7 +795,8 @@ model sklad_suppliers {
   created_at     DateTime? @default(now()) @db.DateTime(0)
   modified_at    DateTime? @db.DateTime(0)
 
-  receipts sklad_receipts[]
+  receipts      sklad_receipts[]
+  issued_orders issued_orders[]
 
   @@map("sklad_suppliers")
 }