feat(invoices): polymorphic billing entity with snapshot clientName

Wires the billingEntityType/billingEntityId columns (added in PR 1) through
the invoice validator and service. Clients can now be billed as either a
client or a company; clientName becomes a snapshot derived from the entity
at create time.

- createInvoiceSchema: replace clientName with billingEntity {type,id}
- listInvoicesSchema: add billingEntityType/billingEntityId filters
- createInvoice: resolveBillingEntity helper (tenant-scoped; tx-aware)
  falls back to entity primary email/address when not supplied
- listInvoices: honor new billing-entity filters
- updateInvoice: unchanged — billing entity is fixed after create
- invoice wizard step 1: temporary billing-entity id input (Task 10.2
  replaces this with a proper picker)

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/lib/validators/invoices.ts

@@ -3,7 +3,10 @@ import { baseListQuerySchema } from '@/lib/api/route-helpers';
 
 export const createInvoiceSchema = z
   .object({
-    clientName: z.string().min(1).max(200),
+    billingEntity: z.object({
+      type: z.enum(['client', 'company']),
+      id: z.string().min(1),
+    }),
     billingEmail: z.string().email().optional(),
     billingAddress: z.string().max(500).optional(),
     dueDate: z.string().min(1),
@@ -35,9 +38,7 @@ export const updateInvoiceSchema = z.object({
   billingEmail: z.string().email().optional(),
   billingAddress: z.string().max(500).optional(),
   dueDate: z.string().min(1).optional(),
-  paymentTerms: z
-    .enum(['immediate', 'net10', 'net15', 'net30', 'net45', 'net60'])
-    .optional(),
+  paymentTerms: z.enum(['immediate', 'net10', 'net15', 'net30', 'net45', 'net60']).optional(),
   currency: z.string().length(3).optional(),
   notes: z.string().max(2000).optional(),
   lineItems: z
@@ -63,6 +64,8 @@ export const listInvoicesSchema = baseListQuerySchema.extend({
   clientName: z.string().optional(),
   dateFrom: z.string().optional(),
   dateTo: z.string().optional(),
+  billingEntityType: z.enum(['client', 'company']).optional(),
+  billingEntityId: z.string().optional(),
 });
 
 export type CreateInvoiceInput = z.infer<typeof createInvoiceSchema>;