fix(build): extract route.ts handlers to handlers.ts (CLAUDE.md convention)

8 API route files were exporting handler functions directly from route.ts,
which Next.js 15 rejects with "$NAME is not a valid Route export field".
Per CLAUDE.md convention, service-tested handler functions live in sibling
handlers.ts files and route.ts only re-exports the GET/POST/etc. wrapped
in withAuth(withPermission(...)).

Discovered during the mobile-foundation Task 24 build validation; the route
files predate this branch but the build was never re-run on data-model.

Files:
- berth-reservations/[id], companies/autocomplete, companies/[id]/members
  + nested mid/set-primary, yachts/autocomplete, yachts/[id]/transfer,
  yachts/[id]/ownership-history
- Integration tests updated to import from handlers.ts (companies,
  memberships, reservations, yachts-detail)

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

tests/integration/api/companies.test.ts

@@ -2,7 +2,7 @@ import { describe, it, expect } from 'vitest';
 
 import { listHandler, createHandler } from '@/app/api/v1/companies/handlers';
 import { getHandler, patchHandler, deleteHandler } from '@/app/api/v1/companies/[id]/handlers';
-import { autocompleteHandler } from '@/app/api/v1/companies/autocomplete/route';
+import { autocompleteHandler } from '@/app/api/v1/companies/autocomplete/handlers';
 import { db } from '@/lib/db';
 import { companies } from '@/lib/db/schema';
 import { eq } from 'drizzle-orm';