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>

messages/en.json

@@ -6782,6 +6782,7 @@
     "back_to_salary": "Back to payroll",
     "payment_date_label": "Payment",
     "payment_date_edit_aria": "Change payment date",
+    "agi_period_note": "AGI is declared for {period} (the payout month)",
     "toast_payment_date_updated": "Payment date updated",
     "header_employees": "{count, plural, one {# employee} other {# employees}}",
     "not_found": "Payroll run not found",

messages/sv.json

@@ -6782,6 +6782,7 @@
     "back_to_salary": "Tillbaka till löner",
     "payment_date_label": "Utbetalning",
     "payment_date_edit_aria": "Ändra utbetalningsdag",
+    "agi_period_note": "AGI redovisas för {period} (utbetalningsmånaden)",
     "toast_payment_date_updated": "Utbetalningsdag uppdaterad",
     "header_employees": "{count, plural, one {# anställd} other {# anställda}}",
     "not_found": "Lönekörning hittades inte",