feat: complete multi-tenant refactor + settings validation + fiscal period API (#156)

* feat: complete multi-tenant refactor for reconciliation, arcim, settings validation

- Migrate bank-reconciliation to company_id (all functions + tests)
- Migrate arcim-migration entity mappers and orchestrator to company_id
- Fix enable-banking reconciliation calls to use companyId
- Add Swedish law validation to settings schema:
  - VAT number required when VAT-registered (ML 11 kap. 8§)
  - Moms period required when VAT-registered (SFL 26 kap.)
  - Aktiebolag must use accrual accounting (BFNAR 2006:1)
- Fix fiscal year period creation: always 12 months after first year (BFL 3 kap.)
- Add plusgiro, website, pays_salaries fields to CompanySettings
- Add plusgiro to invoice PDF template
- Add fiscal period CRUD and opening balances API routes
- Add frame-src CSP directive for future iframe embedding
- Fix unlinked_1930_lines RPC to use company_id parameter
- Update CLAUDE.md documentation

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

* fix: address PR review findings (P1 + P2)

- Fix reconciliation events emitting companyId as userId — thread
  actual userId through runReconciliation and manualLink
- Move VAT cross-field validation (vat_number, moms_period) from
  schema refinements to route handler where effective stored state
  is available, preventing false rejection on partial updates
- Add plusgiro format validation regex (N-N pattern)

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

---------

Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

lib/api/__tests__/schemas.test.ts

@@ -1004,7 +1004,47 @@ describe('UpdateSettingsSchema', () => {
   it('accepts partial update', () => {
     const result = UpdateSettingsSchema.safeParse({
       company_name: 'My AB',
+    })
+    expect(result.success).toBe(true)
+  })
+
+  it('accepts vat_registered: true with required vat_number and moms_period', () => {
+    const result = UpdateSettingsSchema.safeParse({
       vat_registered: true,
+      vat_number: 'SE556123456701',
+      moms_period: 'quarterly',
+    })
+    expect(result.success).toBe(true)
+  })
+
+  it('accepts vat_registered: true without vat_number at schema level (route-level check uses effective state)', () => {
+    const result = UpdateSettingsSchema.safeParse({
+      vat_registered: true,
+      moms_period: 'quarterly',
+    })
+    expect(result.success).toBe(true)
+  })
+
+  it('accepts vat_registered: true without moms_period at schema level (route-level check uses effective state)', () => {
+    const result = UpdateSettingsSchema.safeParse({
+      vat_registered: true,
+      vat_number: 'SE556123456701',
+    })
+    expect(result.success).toBe(true)
+  })
+
+  it('rejects aktiebolag with kontantmetoden (BFNAR 2006:1)', () => {
+    const result = UpdateSettingsSchema.safeParse({
+      entity_type: 'aktiebolag',
+      accounting_method: 'cash',
+    })
+    expect(result.success).toBe(false)
+  })
+
+  it('allows enskild firma with kontantmetoden', () => {
+    const result = UpdateSettingsSchema.safeParse({
+      entity_type: 'enskild_firma',
+      accounting_method: 'cash',
     })
     expect(result.success).toBe(true)
   })