fix(payroll): declare AGI for the payout month, not the run's period month (#2191) (#2228)

Arbetsgivardeklarationen is filed for the calendar month the pay went
out (kontantprincipen), so a run for August paid on 25 September belongs
to redovisningsperiod 202609. The generator, the submit route, the run
page and the run header all took run.period_year/period_month instead,
and three PATCH paths refused any payment date outside that month, which
made lön i efterskott impossible to set up at all.

- lib/salary/agi/reporting-period.ts: one dependency-free helper
  (agiReportingPeriod) derives the period from payment_date, falling
  back to the run period only when the date is missing.
- generate-declaration.ts: XML Redovisningsperiod, the agi_declarations
  lookup/insert and the sanity warnings key on the payout month. New
  AGI_PERIOD_CONFLICT (409) refuses to overwrite another live run's
  declaration for the same payout month; corrections still replace.
- submit route, run page (AGI panel, submission hook, tax-payment fetch,
  XML filename) and RunHeader use the helper; the header says "AGI
  redovisas för 2026-09 (utbetalningsmånaden)" whenever the two differ.
- The in-period payment-date guard is lifted in the dashboard PATCH,
  lib/salary/update-run.ts (MCP staged tool + pending-ops executor) and
  the v1 PATCH, plus the RunHeader min/max; its only stated reason was
  the period-keyed AGI. Generated API skill reference updated.

Existing agi_declarations rows keep their stored period: a declaration
already filed under the earned month is a correction with Skatteverket,
not a re-key. Rule verified against Skatteverket's guidance on
redovisningsperiod (kontantprincipen).

Closes #2191


Claude-Session: https://claude.ai/code/session_01QPQLwHNEiQfiCNLSMzXMiQ

Co-authored-by: Jakob Wennberg <311770904+jakobwennberg-oss@users.noreply.github.com>
Co-authored-by: Claude Fable 5.1 <noreply@anthropic.com>

lib/pending-operations/__tests__/payroll-executors.test.ts

@@ -249,11 +249,15 @@ describe('commitPendingOperation: update_salary_run', () => {
     expect(findCall('salary_run_employees', 'update')).toEqual([{ calculation_breakdown: null }])
   })
 
-  it('rejects a payment_date outside the run period month with 400', async () => {
+  it('commits a payment_date in the month after the period (lön i efterskott, #2191)', async () => {
+    // The AGI redovisningsperiod follows payment_date, so a March run paid
+    // in April is declared for April; no period guard refuses it.
     const { supabase, enqueue } = createQueuedMockSupabase()
     enqueue({ data: { id: 'op-1' }, error: null }) // CAS claim
     enqueue({ data: RUN_ROW }) // draft gate passes; period is 2026-03
-    enqueue({ data: null, error: null }) // finalize (rejected)
+    enqueue({ data: { ...RUN_ROW, payment_date: '2026-04-05' } }) // optimistic-locked update
+    enqueue({ data: null, error: null }) // roster calculation_breakdown clear
+    enqueue({ data: null, error: null }) // finalize
 
     const op = makePendingOp({
       operation_type: 'update_salary_run',
@@ -261,8 +265,8 @@ describe('commitPendingOperation: update_salary_run', () => {
     })
     const result = await commitPendingOperation(supabase as never, 'user-1', 'company-1', op)
 
-    expect(result.status).not.toBe('committed')
-    expect(result.http_status).toBe(400)
+    expect(result.status).toBe('committed')
+    expect(result.data).toMatchObject({ payment_date: '2026-04-05' })
   })
 
   it('fails when the calculation-invalidation clear errors (guard is not best-effort)', async () => {