docs: drop remaining stale MassImportService/ExcelService references

Replace the legacy raw-spreadsheet importer references left behind after
#674 with the canonical import architecture (CanonicalImportOrchestrator +
four loaders) and document #686 index-based PDF resolution.

- l3-backend-3b: DocumentImporter now resolves PDF by index (importDir/
  <index>.pdf) with index validation + canonical-path containment + %PDF
  magic-byte check (no recursive walk / homoglyph file-path guards)
- c4-diagrams.md: replace massImport/excelSvc components + their rels with
  an importOrch (CanonicalImportOrchestrator) component wired to doc/person/
  tag services; refresh adminCtrl and adminSystem descriptions
- ARCHITECTURE.md: importing package row now describes the orchestrator +
  four loaders consuming canonical artifacts
- TODO-backend.md: remove obsolete "MassImportService provides no status"
  item (service deleted; orchestrator already exposes import-status); update
  stale ExcelService test-coverage suggestion

Refs #686

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

docs/ARCHITECTURE.md

@@ -65,7 +65,7 @@ Members of the cross-cutting layer have no entity of their own, no user-facing C
 | `dashboard` | Stats aggregation for the admin dashboard and Family Pulse widget | Aggregates from 3+ domains; no owned entities |
 | `exception` | `DomainException`, `ErrorCode` enum, `GlobalExceptionHandler` | Framework infra; consumed by every controller and service. Adding a new `ErrorCode` requires matching updates in `frontend/src/lib/shared/errors.ts` and all three `messages/*.json` locale files. Current security-related codes: `CSRF_TOKEN_MISSING` (403 on mutating request without valid `X-XSRF-TOKEN` header), `TOO_MANY_LOGIN_ATTEMPTS` (429 when login rate limit exceeded). |
 | `filestorage` | `FileService` — MinIO/S3 upload, download, presigned-URL generation | Generic service; consumed by `document` and `ocr` |
-| `importing` | `MassImportService` — async ODS/Excel batch import | Orchestrates across `person`, `tag`, `document` |
+| `importing` | `CanonicalImportOrchestrator` — async canonical import running four idempotent loaders (`TagTreeImporter` → `PersonRegisterImporter` → `PersonTreeImporter` → `DocumentImporter`) over the normalizer's committed canonical artifacts (`canonical-*.xlsx` + `canonical-persons-tree.json`) | Orchestrates across `person`, `tag`, `document` |
 | `security` | `SecurityConfig`, `Permission` enum, `@RequirePermission` annotation, `PermissionAspect` (AOP) | Framework infra; enforced globally across all controllers |
 
 **Frontend `shared/`** follows the same admission criteria. Key members: `api.server.ts` (typed openapi-fetch client factory), `errors.ts` (backend `ErrorCode` → i18n mapping), `shared/primitives/` (generic UI components used across ≥2 domains), `shared/discussion/` (comment/mention editor used by `document` and `geschichte`), `shared/utils/` (pure date/sort/debounce utilities).