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>

tests/helpers.ts

@@ -495,6 +495,8 @@ export function makeCompanySettings(
     country: 'SE',
     phone: null,
     email: null,
+    website: null,
+    pays_salaries: false,
     f_skatt: true,
     vat_registered: true,
     vat_number: null,
@@ -505,6 +507,7 @@ export function makeCompanySettings(
     clearing_number: null,
     account_number: null,
     bankgiro: null,
+    plusgiro: null,
     iban: null,
     bic: null,
     accounting_method: 'accrual',