feat(company-memberships): service + validators + tests

Adds company-membership service with six operations (add, update, end,
setPrimary, listByCompany, listByClient), the corresponding Zod
validators, three socket events, and a unit-test suite covering the
portId-scoping rules, the unique_cm_exact conflict path, and the atomic
setPrimary transaction.

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

tests/unit/validators.test.ts

@@ -7,6 +7,7 @@ import { createWebhookSchema, updateWebhookSchema } from '@/lib/validators/webho
 import { createFieldSchema, updateFieldSchema } from '@/lib/validators/custom-fields';
 import { createYachtSchema, transferOwnershipSchema } from '@/lib/validators/yachts';
 import { createCompanySchema } from '@/lib/validators/companies';
+import { addMembershipSchema } from '@/lib/validators/company-memberships';
 
 // ─── Client schemas ───────────────────────────────────────────────────────────
 
@@ -454,3 +455,31 @@ describe('createCompanySchema', () => {
     expect(result.success).toBe(true);
   });
 });
+
+// ─── Company membership schemas ──────────────────────────────────────────────
+
+describe('addMembershipSchema', () => {
+  const validInput = {
+    clientId: 'client-uuid-1',
+    role: 'director' as const,
+    startDate: '2026-01-01',
+  };
+
+  it('rejects missing clientId', () => {
+    const result = addMembershipSchema.safeParse({
+      role: 'director',
+      startDate: '2026-01-01',
+    });
+    expect(result.success).toBe(false);
+  });
+
+  it('rejects invalid role', () => {
+    const result = addMembershipSchema.safeParse({ ...validInput, role: 'janitor' });
+    expect(result.success).toBe(false);
+  });
+
+  it('accepts minimal valid input', () => {
+    const result = addMembershipSchema.safeParse(validInput);
+    expect(result.success).toBe(true);
+  });
+});