fix(invoices): guard cash-method partial payments across every payment path (#1413)

* fix(invoices): guard cash-method partial payments across every payment path

A never-booked kontantmetoden invoice can only be settled by the generated
cash entry (createInvoiceCashEntry / createSupplierInvoiceCashEntry), and
that entry always books the FULL invoice: it takes no payment amount. Three
payment surfaces still let partial payments through to it, corrupting books:

- settleInvoicePayment dropped the fully-paid term entirely, so a partial
  payment (Stripe sync, mark-paid) booked the entire invoice: over-recognized
  revenue, over-declared output VAT, and a bank debit that did not match the
  money received.
- The dashboard and agent match-transaction paths fell back to an
  accrual-style clearing entry against an EMPTY 1510: negative receivable,
  no revenue, no moms (ML 13 kap 8 § puts each installment's moms in its own
  receipt period). The comment claimed the credit "gets resolved on final
  payment", but the cash builder never touches 1510 and books the full
  total, so the final payment double-debited the bank instead.
- The supplier routes had no full-settlement term at all, so a partial
  payment booked the full expense + input VAT.

Fix: one shared predicate (cashPartialBlockReason in booking-mode.ts)
rejects generated cash entries unless the payment settles the invoice in
full from a fully unpaid state, wired into all six POST surfaces, the agent
commit paths, and the three preview routes (so dialogs cannot propose a
verifikat the POST refuses). New bilingual error codes
INVOICE_PAID_CASH_PARTIAL_UNSUPPORTED / SI_CASH_PARTIAL_UNSUPPORTED.
Invoices booked at issue are unaffected: their partial payments keep the
normal 1510/2440 clearing path.

The v1 match-invoice route already had this guard (VALIDATION_ERROR); its
behavior is unchanged. Proper per-installment recognition (proportional
revenue + moms per receipt) is the follow-up feature that would lift this
restriction.

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

* docs(review): drop stale ML 13 kap 8 § cites for kontantmetoden VAT timing

Compliance-review finding: the section is the old ML 1994:200 numbering; in
ML 2023:200, 13 kap covers input-VAT deduction, not redovisningstidpunkt.
The substantive rule (bokslutsmetoden reports moms at payment, per
installment, except at year-end) is unchanged and stated without a section
cite until the current-law section is verified. Comments and cookbook prose
only; no behavior change. Also fixes the two pre-existing occurrences.

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

---------

Co-authored-by: Jakob Wennberg <311770904+jakobwennberg-oss@users.noreply.github.com>
Co-authored-by: Claude Fable 5 <noreply@anthropic.com>

lib/docs/content/cookbook/ingest-bank-transactions.ts

@@ -214,7 +214,7 @@ The ledger is SEK-denominated; foreign amounts are converted to SEK automaticall
 
 - **Re-running the same file is safe; date-only overlap is also safe.** Dedup is primarily by a stable \`external_id\` (date + amount + counterparty); a secondary content match on \`(date, amount, description)\` against already-booked rows also skips, so partial overlap of two statements doesn't double-import.
 - **Settlement account selection matters.** The wrong settlement account silently breaks bank reconciliation later. \`1930\` (företagskonto) is the SEK default; a foreign-currency bank account uses its own asset account (e.g. \`1932\` for USD). \`/imports/bank\` resolves the settlement account automatically — to set it explicitly, ingest via \`POST /transactions/ingest\` with \`settlement_account\`.
-- **Cash-method companies and partial payments don't mix.** If \`company_settings.accounting_method = 'cash'\` and you try to match a partial payment, the response is \`VALIDATION_ERROR\` rather than booking accrual entries: cash-method cannot model the per-installment moms event correctly (ML 13 kap 8 §). Either book the partial payment as a separate categorisation or switch to accrual.
+- **Cash-method companies and partial payments don't mix.** If \`company_settings.accounting_method = 'cash'\` and you try to match a partial payment, the response is \`VALIDATION_ERROR\` rather than booking accrual entries: cash-method cannot model the per-installment moms event correctly (bokslutsmetoden reports moms at payment, per installment). Either book the partial payment as a separate categorisation or switch to accrual.
 - **Batch-categorize is partial-success by default.** If one item hits a locked period, the others still commit. The summary block tells you the totals; check per-item \`ok\` flags.
 
 ## Next steps