feat(vat): oss momskod keeps unionsordningen sales out of the momsdeklaration (#1797)

A Fortnox user with OSS sales hit the SIE import mapping step and found no
way to map OSS accounts: the momskod picker had no OSS option, 3106-style
labels ("Försäljning varor till annat EU-land, momspliktig") were suggested
as EU-varor (ruta 35), and an OSS revenue account with a sats set leaked into
ruta 05. Skatteverket: "Den försäljning som du redovisar i OSS ska du inte
redovisa i den vanliga momsdeklarationen."

- add the 'oss' revenue treatment: allowed for class 3 only, mapped to no
  ruta, default rate null (destination-country rate is not a Swedish sats);
  explicit 'oss' also overrides static BAS mappings such as 3001
- REVENUE_RUTA becomes a partial map where null = allowed but off the
  declaration, so the class gate no longer conflates "no ruta" with
  "purchase-only"
- SIE label suggestion: OSS/unionsordningen labels suggest 'oss';
  momspliktig EU-varor labels are left for review instead of ruta 35
- AccountVatTreatmentSchema derives from ACCOUNT_VAT_TREATMENTS instead of a
  second literal list
- migration widens the class-aware CHECK with 'oss' for class 3 (superset;
  NOT VALID + VALIDATE like its predecessor); pg test extended
- sv/en labels; unit tests for resolver, suggestion, declaration exclusion

Per-country VAT rates on invoices and the quarterly EUR/ECB OSS underlag
remain unbuilt (DECISIONS.md).


Claude-Session: https://claude.ai/code/session_01E3QB8GxJ9tS217agHjLRk7

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

supabase/migrations/20260822093000_account_vat_treatment_oss.sql

@@ -0,0 +1,38 @@
+-- OSS (unionsordningen): sales to EU consumers above the EUR 10 000 threshold
+-- carry the destination country's VAT and are declared only in the quarterly
+-- OSS declaration, never in the Swedish momsdeklaration. The 'oss' revenue
+-- treatment keeps such accounts out of every ruta; the resolver lives in
+-- lib/vat/account-vat-treatment.ts. Purchase classes are unchanged.
+ALTER TABLE public.chart_of_accounts
+  DROP CONSTRAINT IF EXISTS chart_of_accounts_default_vat_treatment_check;
+
+ALTER TABLE public.chart_of_accounts
+  ADD CONSTRAINT chart_of_accounts_default_vat_treatment_check
+  CHECK (
+    default_vat_treatment IS NULL
+    OR (
+      account_class = 3
+      AND default_vat_treatment IN (
+        'standard_25', 'reduced_12', 'reduced_6', 'exempt',
+        'reverse_charge_domestic', 'reverse_charge_eu_goods',
+        'reverse_charge_eu_services', 'export_goods', 'export_services',
+        'vmb', 'rental_voluntary', 'oss'
+      )
+    )
+    OR (
+      account_class BETWEEN 4 AND 6
+      AND default_vat_treatment IN (
+        'reverse_charge_domestic', 'reverse_charge_eu_goods',
+        'reverse_charge_eu_services', 'reverse_charge_non_eu_services'
+      )
+    )
+  ) NOT VALID;
+
+-- Superset of the previous constraint: validation cannot fail on existing rows.
+ALTER TABLE public.chart_of_accounts
+  VALIDATE CONSTRAINT chart_of_accounts_default_vat_treatment_check;
+
+COMMENT ON COLUMN public.chart_of_accounts.default_vat_treatment IS
+  'Per-account momsdeklaration treatment. Explicit values override the built-in BAS account mapping; ''oss'' keeps revenue out of the Swedish declaration (declared in OSS).';
+
+NOTIFY pgrst, 'reload schema';

supabase/migrations/__tests__/account-default-vat-treatment.pg.test.ts

@@ -63,7 +63,7 @@ describe('chart_of_accounts.default_vat_treatment', () => {
       'standard_25', 'reduced_12', 'reduced_6', 'exempt',
       'reverse_charge_domestic', 'reverse_charge_eu_goods',
       'reverse_charge_eu_services', 'export_goods', 'export_services',
-      'vmb', 'rental_voluntary', null,
+      'vmb', 'rental_voluntary', 'oss', null,
     ]
     for (const treatment of revenueTreatments) {
       await expect(setTreatment(companyId, '3001', treatment)).resolves.toBeDefined()
@@ -89,6 +89,14 @@ describe('chart_of_accounts.default_vat_treatment', () => {
     await expect(setTreatment(companyId, '4010', 'standard_25')).rejects.toThrow()
   })
 
+  it('accepts oss only on revenue accounts (20260822093000_account_vat_treatment_oss)', async () => {
+    const { companyId, userId } = await seedCompany()
+    await insertAccount(companyId, userId, '3106', 3)
+    await insertAccount(companyId, userId, '4010', 4)
+    await expect(setTreatment(companyId, '3106', 'oss')).resolves.toBeDefined()
+    await expect(setTreatment(companyId, '4010', 'oss')).rejects.toThrow()
+  })
+
   it('normalizes values accepted by the predecessor before enforcing classes', async () => {
     const { companyId, userId } = await seedCompany()
     await insertAccount(companyId, userId, '1010', 1)