feat(deadlines): gate AGI on employer registration + stop completing AGI at XML generation (#1062)

* feat(deadlines): gate F-skatt reminders on debited preliminary tax, add durable dismissal

The f_skatt deadline was gated on the F-skatt approval flag (DB default
true), giving nearly every company 12 monthly payment reminders for a tax
Skatteverket may not have debited at all (64% of all system deadline rows,
one lifetime completion). Approval carries no recurring obligation; the
monthly duty is payment of debiterad preliminarskatt and exists only while
the debited amount is > 0 (SFL 62 kap. 4-5 par., 55 kap. 2 par.).

- Gate the f_skatt deadline on preliminary_tax_monthly > 0 (field already
  collected at onboarding, previously unread) and retitle it as a payment.
- Storforetag keep the 12th in August (January-only 17th, 62 kap. 3 par.).
- Declare the prod-only preliminary_tax_monthly column in a migration so
  installs built purely from migrations stop failing tax-settings saves.
- Add deadlines.dismissed_at: DELETE on a system deadline now soft-dismisses
  it durably (hard deletes were resurrected by the nightly backfill within
  24h); generator, backfill, and every read surface respect it.
- Prune upcoming f_skatt rows for companies with no debited amount.

Closes part of #1028.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

* feat(deadlines): gate AGI on employer registration, stop completing AGI deadline at XML generation

The arbetsgivardeklaration deadline was gated on pays_salaries, which is
wrong in both directions: a registered employer must file AGI every month
including nil months (SFL 26 kap. 3 par.), and companies actively running
payroll with the flag off got no AGI reminders at all (each missed monthly
filing risks a forseningsavgift).

- New company_settings.employer_registered (nullable, no default) gates
  AGI and the storforetag skatteinbetalning row; pays_salaries remains a
  fallback for rows saved before the flag existed and keeps its UI meaning.
- Migration backfills employer_registered=true from pays_salaries=true and
  from actual payroll activity (salary_runs).
- New employer_seasonal flag: sasongsregistrerade file only for payment
  months plus a December nil declaration, so only the December-period row
  is generated.
- Settings UI: registration + seasonal checkboxes (sv/en strings).
- AGI XML generation no longer auto-completes the deadline as submitted:
  SFL 26 kap. deems the obligation satisfied only when the declaration has
  come in to Skatteverket. The Skatteverket extension's kvittens reconcile
  remains the confirming path; manual filers tick the deadline themselves.

Part of #1028.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

* fix(deadlines): include dismissed_at in DeadlineForm payload

The Deadline type gained the required dismissed_at field; the form's
submit payload literal must carry it for the Omit<Deadline, ...> shape.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

* fix(deadlines): make system-deadline dismissal atomic

Constrain the dismiss update to source='system' and verify a row was
actually updated: a concurrent regeneration can delete the row between
lookup and update, and the route must not report a phantom success.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

---------

Co-authored-by: Claude Fable 5 <noreply@anthropic.com>

messages/en.json

@@ -1777,7 +1777,11 @@
     "month_nov": "November",
     "month_dec": "December",
     "pays_salaries_label": "Pays salaries",
-    "pays_salaries_help": "Affects which tax deadlines are shown (employer declarations, etc.).",
+    "pays_salaries_help": "Controls the salary module in the menu. Paying out salary requires employer registration with Skatteverket.",
+    "employer_registered_label": "Registered as employer",
+    "employer_registered_help": "Registered employers must file an employer declaration (AGI) every month, including months without salary payments (nil declaration). Controls AGI deadlines.",
+    "employer_seasonal_label": "Seasonal employer (säsongsregistrerad)",
+    "employer_seasonal_help": "Files the employer declaration only for months with salary payments, plus December if no salary was paid during the year.",
     "preliminary_tax_heading": "Preliminary tax",
     "preliminary_tax_monthly_label": "Monthly preliminary tax (F-skatt)",
     "preliminary_tax_monthly_help": "Monthly amount per Skatteverket's debited preliminary tax decision. Payment reminders are only created when an amount is set. Leave empty if no preliminary tax is debited."

messages/sv.json

@@ -1777,7 +1777,11 @@
     "month_nov": "November",
     "month_dec": "December",
     "pays_salaries_label": "Betalar löner",
-    "pays_salaries_help": "Påverkar vilka skattedeadlines som visas (arbetsgivardeklaration m.m.).",
+    "pays_salaries_help": "Styr lönemodulen i menyn. Att betala ut lön kräver arbetsgivarregistrering hos Skatteverket.",
+    "employer_registered_label": "Registrerad som arbetsgivare",
+    "employer_registered_help": "Registrerade arbetsgivare ska lämna arbetsgivardeklaration varje månad, även månader utan löneutbetalning (nolldeklaration). Styr AGI-deadlines.",
+    "employer_seasonal_label": "Säsongsregistrerad arbetsgivare",
+    "employer_seasonal_help": "Lämnar arbetsgivardeklaration bara för månader med löneutbetalning, samt för december om ingen lön betalats under året.",
     "preliminary_tax_heading": "Preliminärskatt",
     "preliminary_tax_monthly_label": "Månatlig preliminärskatt (F-skatt)",
     "preliminary_tax_monthly_help": "Månadsbelopp enligt Skatteverkets beslut om debiterad preliminärskatt. Betalningspåminnelser skapas bara när ett belopp är angivet. Lämna tomt om ingen preliminärskatt är debiterad."