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>

lib/api/schemas.ts

@@ -1467,6 +1467,8 @@ export const UpdateSettingsSchema = z.object({
   tax_contact_email: z.string().email().nullable().optional().or(z.literal('')),
   fiscal_year_start_month: z.number().int().min(1).max(12).optional(),
   preliminary_tax_monthly: z.number().nullable().optional(),
+  employer_registered: z.boolean().nullable().optional(),
+  employer_seasonal: z.boolean().optional(),
   bank_name: z.string().max(100, 'Banknamn får vara max 100 tecken').optional(),
   clearing_number: z.string().regex(/^\d{4,5}$/, 'Clearingnummer måste vara 4-5 siffror').optional().or(z.literal('')),
   account_number: z.string().regex(/^\d{6,12}$/, 'Kontonummer måste vara 6-12 siffror').optional().or(z.literal('')),