docs(importing): document the canonical importer rebuild

- ADR-025: add decision 3 (four idempotent loaders over canonical artifacts;
  raw spreadsheet no longer parsed by Java) with the settled Option-A name
  policy, human-edit-preserve precedence, provisional contract, and ported
  security guards.
- l3-backend-3b diagram: replace MassImportService/ExcelService with the
  orchestrator, the four loaders, and CanonicalSheetReader, with the loader
  dependency edges.
- GLOSSARY: Canonical import / canonical artifact / CanonicalSheetReader terms;
  refresh SkippedFile (new INVALID_FILENAME_PATH_TRAVERSAL reason, index key).
- DEPLOYMENT §6: canonical-artifact prerequisite runbook (run normalizer →
  place four artifacts → trigger import); note idempotent re-run.
- CLAUDE.md (root + backend): importing/ package now lists the orchestrator +
  loaders + CanonicalSheetReader.

OpenAPI: no generate:api needed — the ImportStatus/SkippedFile generated
schemas already match the new types byte-for-byte (same fields + SkipReason
enum), so the API surface is unchanged.

Closes #669

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

backend/CLAUDE.md

@@ -34,7 +34,7 @@ src/main/java/org/raddatz/familienarchiv/
 ├── exception/           # DomainException, ErrorCode, GlobalExceptionHandler
 ├── filestorage/         # FileService (S3/MinIO)
 ├── geschichte/          # Geschichte (story) domain
-├── importing/           # MassImportService
+├── importing/           # CanonicalImportOrchestrator + 4 loaders + CanonicalSheetReader
 ├── notification/        # Notification domain + SseEmitterRegistry
 ├── ocr/                 # OCR domain — OcrService, OcrBatchService, training
 ├── person/              # Person domain — Person, PersonService, PersonController