Remove multi-tenant company_id scoping from entire codebase

Drop company_id column from all 22 domain tables via migration.
Remove companyId from JWT payload, auth plugins, all service method
signatures (~215 occurrences), all route handlers (~105 occurrences),
test runner, test suites, and frontend auth store/types.

The company table stays as store settings (name, timezone). Tenant
isolation in a SaaS deployment would be at the database level (one
DB per customer) not the application level.

All 107 API tests pass. Zero TSC errors across all packages.

packages/backend/__tests__/routes/v1/accounts.test.ts

@@ -45,7 +45,6 @@ describe('Account routes', () => {
       const body = response.json()
       expect(body.name).toBe('Smith Family')
       expect(body.email).toBe('smith@example.com')
-      expect(body.companyId).toBe(TEST_COMPANY_ID)
       expect(body.billingMode).toBe('consolidated')
     })