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>

src/lib/validators/company-memberships.ts

@@ -0,0 +1,36 @@
+import { z } from 'zod';
+
+export const ROLES = [
+  'director',
+  'officer',
+  'broker',
+  'representative',
+  'legal_counsel',
+  'employee',
+  'shareholder',
+  'other',
+] as const;
+
+export const addMembershipSchema = z.object({
+  clientId: z.string().min(1),
+  role: z.enum(ROLES),
+  roleDetail: z.string().optional(),
+  startDate: z.coerce.date(),
+  isPrimary: z.boolean().optional().default(false),
+  notes: z.string().optional(),
+});
+
+export const updateMembershipSchema = z.object({
+  role: z.enum(ROLES).optional(),
+  roleDetail: z.string().nullable().optional(),
+  notes: z.string().nullable().optional(),
+  isPrimary: z.boolean().optional(),
+});
+
+export const endMembershipSchema = z.object({
+  endDate: z.coerce.date(),
+});
+
+export type AddMembershipInput = z.infer<typeof addMembershipSchema>;
+export type UpdateMembershipInput = z.infer<typeof updateMembershipSchema>;
+export type EndMembershipInput = z.infer<typeof endMembershipSchema>;