fix(transactions): derive pre-migration marker cutoff from imported voucher dates, not fiscal_year_end (#2047)

* fix(transactions): derive pre-migration marker cutoff from imported voucher dates, not fiscal_year_end

A SIE file exported mid-year still declares the full fiscal year in #RAR 0,
so sie_imports.fiscal_year_end is a future date for mid-year migrators and
the 'fran perioden fore din migrering' marker fired on every new bank
transaction until New Year. The cutoff now comes from the latest posted
source_type='import' entry_date (excluding the M-series omforingsverifikation,
which is deliberately dated at fiscal year end), so it tracks where the
imported bokforing actually ends and self-corrects on undo/replace.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01UU5QbL3p8xNB3tSZDbyrr3

* fix(transactions): arm pre-migration cutoff on completed SIE import, exclude omforing by description

Skeptic findings on the frozen commit: (1) source_type='import' is accepted
from v1 API clients, so a never-migrated company with an API-labeled backfill
would get a false cutoff; the marker is now armed only when a completed
sie_imports row exists. (2) Imported vouchers keep the source file's voucher
series, so excluding series M dropped genuine M-series vouchers (6 prod
companies); the omforingsverifikation is now excluded by its hardcoded
description prefix instead.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01UU5QbL3p8xNB3tSZDbyrr3

* docs(transactions): state the pre-migration cutoff residuals truthfully

The ledger duplicate guard only reaches the completed-year single-skip case
(7-day window vs a fiscal-year-end-dated aggregate omforing), so it is not
a general backstop for the skip-window gap; the gap is accepted on rarity.
Also documents the rattelse-rename fragility of the description-keyed
exclusion. Skeptic re-review condition, no code change.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01UU5QbL3p8xNB3tSZDbyrr3

---------

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

app/(dashboard)/transactions/page.tsx

@@ -76,6 +76,7 @@ import { cn, formatCurrency, formatDate } from '@/lib/utils'
 import { roundOre } from '@/lib/money'
 import type { TransactionCategory, CreateTransactionInput, Invoice, Customer, SupplierInvoice, Supplier, VatTreatment, EntityType, LinePatternEntry, BookingTemplateLibrary } from '@/types'
 import type { SuggestedTemplate } from '@/lib/transactions/category-suggestions'
+import { fetchMigrationCoverageEnd } from '@/lib/transactions/migration-coverage'
 import { isImportedTransaction } from '@/lib/transactions/origin'
 import { computeJeUnderlagStatus, type JeUnderlagStatus } from '@/lib/transactions/underlag-status'
 import { isWithinBounds, resolvePeriodBounds } from '@/lib/transactions/period-filter'
@@ -560,10 +561,13 @@ export default function TransactionsPage() {
   // "Kör matchning igen" in the review surface.
   const [rerunningMatch, setRerunningMatch] = useState(false)
 
-  // End of the company's completed SIE-import coverage (latest
-  // fiscal_year_end). Drives the quiet "från perioden före din migrering"
-  // marker on inbox rows: period-based on purpose, it labels which period a
-  // row belongs to, it never suggests a sync skip date (that was #917).
+  // End of the company's SIE-migration data coverage (latest imported
+  // voucher date, see lib/transactions/migration-coverage.ts). Drives the
+  // quiet "från perioden före din migrering" marker on inbox rows: date-based
+  // on purpose, it labels rows the imported bokföring should already cover,
+  // it never suggests a sync skip date (that was #917). Not fiscal_year_end:
+  // for a mid-year migration that is a future date and the marker fired on
+  // every new transaction until New Year.
   const [sieCoverageEnd, setSieCoverageEnd] = useState<string | null>(null)
   useEffect(() => {
     if (!companyId) {
@@ -572,17 +576,9 @@ export default function TransactionsPage() {
     }
     let cancelled = false
     ;(async () => {
-      const { data } = await supabase
-        .from('sie_imports')
-        .select('fiscal_year_end')
-        .eq('company_id', companyId)
-        .eq('status', 'completed')
-        .not('fiscal_year_end', 'is', null)
-        .order('fiscal_year_end', { ascending: false })
-        .limit(1)
-        .maybeSingle()
+      const coverageEnd = await fetchMigrationCoverageEnd(supabase, companyId)
       if (!cancelled) {
-        setSieCoverageEnd((data as { fiscal_year_end?: string } | null)?.fiscal_year_end || null)
+        setSieCoverageEnd(coverageEnd)
       }
     })()
     return () => {