feat(deadlines): gate F-skatt reminders on debited preliminary tax + durable dismissal (#1057)

* 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>

* 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

@@ -1780,7 +1780,7 @@
     "pays_salaries_help": "Affects which tax deadlines are shown (employer declarations, etc.).",
     "preliminary_tax_heading": "Preliminary tax",
     "preliminary_tax_monthly_label": "Monthly preliminary tax (F-skatt)",
-    "preliminary_tax_monthly_help": "Amount in SEK paid each month."
+    "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."
   },
   "settings_backup_download": {
     "create_backup_title": "Create backup",

messages/sv.json

@@ -1780,7 +1780,7 @@
     "pays_salaries_help": "Påverkar vilka skattedeadlines som visas (arbetsgivardeklaration m.m.).",
     "preliminary_tax_heading": "Preliminärskatt",
     "preliminary_tax_monthly_label": "Månatlig preliminärskatt (F-skatt)",
-    "preliminary_tax_monthly_help": "Belopp i SEK som betalas varje månad."
+    "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."
   },
   "settings_backup_download": {
     "create_backup_title": "Skapa backup",