fix(salary): dedicated migration + tests for salary models (W2-B-038) - #947

…038)

W2-B-038 — salaryBatch / salaryItem / salarySchedule models missing

Root cause
----------
The Prisma schema already defined SalaryBatch, SalaryItem, and SalarySchedule,
but the three salary tables were bundled into the large 20260423111042_init
migration without a standalone, clearly-named migration of their own.
Environments that ran a subset of migrations (or fresh deploys that need an
audit trail of when each feature was added) had no dedicated migration to apply,
causing prisma migrate deploy to leave the tables absent and TypeScript to
report TS2339 on every prisma.salaryBatch / salaryItem / salarySchedule access
(13 sites in salaryService.ts).

Changes
-------
prisma/migrations/20260901000000_add_salary_models/migration.sql
  - Creates salary_batches, salary_items, salary_schedules with full column
    definitions matching schema.prisma exactly.
  - All CREATE TABLE statements use IF NOT EXISTS so the migration is safe to
    run on environments that already have the tables from the init migration.
  - Unique and regular indexes use CREATE INDEX IF NOT EXISTS.
  - Foreign keys are added inside a DO dollar block that checks pg_constraint
    first, making them idempotent too.

tests/salaryService.test.ts (20 test cases, no real DB)
  - createSalaryBatch: normal path, idempotency hit, total mismatch (400),
    correct total accepted
  - processSalaryBatch: all-success (completed), partial failure
    (partially_completed), all-failed, resume support (skip completed items),
    rejected transfer writes via dollar-transaction, batch-not-found guard,
    already-completed guard
  - getSalaryBatches: pagination params forwarded, includes count.items
  - createSalarySchedule: normal path + nextRunAt from dateUtils, invalid
    cron (400), empty cron (400), currency default
  - triggerSchedule: fires createSalaryBatch, updates lastRunAt/nextRunAt,
    skips non-active schedules, uses 60s offset for non-daily cron