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>

tests/unit/validators.test.ts

@@ -194,7 +194,7 @@ describe('updateBerthStatusSchema', () => {
 
 describe('createInvoiceSchema', () => {
   const validInvoice = {
-    clientName: 'Bob',
+    billingEntity: { type: 'client' as const, id: 'client-123' },
     dueDate: '2026-06-01',
     lineItems: [{ description: 'Berth fee', quantity: 1, unitPrice: 5000 }],
   };
@@ -203,9 +203,17 @@ describe('createInvoiceSchema', () => {
     expect(createInvoiceSchema.safeParse(validInvoice).success).toBe(true);
   });
 
+  it('accepts a valid invoice with billingEntity type=company', () => {
+    const result = createInvoiceSchema.safeParse({
+      ...validInvoice,
+      billingEntity: { type: 'company' as const, id: 'company-123' },
+    });
+    expect(result.success).toBe(true);
+  });
+
   it('accepts invoice with only expenseIds', () => {
     const result = createInvoiceSchema.safeParse({
-      clientName: 'Bob',
+      billingEntity: { type: 'client' as const, id: 'client-123' },
       dueDate: '2026-06-01',
       expenseIds: ['exp-1'],
     });
@@ -213,12 +221,34 @@ describe('createInvoiceSchema', () => {
   });
 
   it('rejects invoice with neither lineItems nor expenseIds', () => {
-    const result = createInvoiceSchema.safeParse({ clientName: 'Bob', dueDate: '2026-06-01' });
+    const result = createInvoiceSchema.safeParse({
+      billingEntity: { type: 'client' as const, id: 'client-123' },
+      dueDate: '2026-06-01',
+    });
     expect(result.success).toBe(false);
   });
 
-  it('rejects empty clientName', () => {
-    const result = createInvoiceSchema.safeParse({ ...validInvoice, clientName: '' });
+  it('rejects missing billingEntity', () => {
+    const result = createInvoiceSchema.safeParse({
+      dueDate: '2026-06-01',
+      lineItems: [{ description: 'Fee', quantity: 1, unitPrice: 1 }],
+    });
+    expect(result.success).toBe(false);
+  });
+
+  it('rejects billingEntity with invalid type', () => {
+    const result = createInvoiceSchema.safeParse({
+      ...validInvoice,
+      billingEntity: { type: 'unknown', id: 'id-1' },
+    });
+    expect(result.success).toBe(false);
+  });
+
+  it('rejects billingEntity with empty id', () => {
+    const result = createInvoiceSchema.safeParse({
+      ...validInvoice,
+      billingEntity: { type: 'client', id: '' },
+    });
     expect(result.success).toBe(false);
   });