Bug/year end failure (#575)

* feat: implement findNextPeriod function and integrate into year-end closing logic

* feat: add integrity check for PDF documents and enhance user feedback for corrupt files

* Refactor year-end service and period creation logic for improved UTC handling and error messaging

- Update `validateYearEndReadiness` to assert on stable warning messages without interpolating period names.
- Modify `createNextPeriod` to ensure date calculations are performed in UTC, preventing DST-related issues.
- Enhance error handling in `validateYearEndReadiness` and `executeYearEndClosing` to avoid exposing database details.
- Introduce structured error messages for year-end processes in `structured-errors.ts`.
- Add tests for document integrity checks, ensuring proper authentication and error handling.
- Implement GUC checks in document versioning to prevent unauthorized modifications and ensure company membership.
- Update migration scripts to reflect changes in document immutability enforcement.

* fix: add comment to clarify GUC behavior in document supersession logic

lib/errors/structured-errors.ts

@@ -770,6 +770,11 @@ const YEAR_END: Record<string, StructuredErrorEntry> = {
     message_sv: 'Resultaträkningens debet och kredit balanserar inte. Granska verifikationerna innan bokslut.',
     message_en: 'Trial balance does not balance.',
   },
+  YEAR_END_NEXT_PERIOD_HAS_IB: {
+    httpStatus: 400,
+    message_sv: 'Nästa räkenskapsperiod har redan ingående balanser bokförda. Storno dem innan du kör om bokslutet.',
+    message_en: 'Next fiscal period already has opening balances posted; reverse them before re-running year-end.',
+  },
 }
 
 const OPENING_BAL: Record<string, StructuredErrorEntry> = {