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/c4/l3-backend-3b-document-management.puml

@@ -16,7 +16,7 @@ System_Boundary(backend, "API Backend (Spring Boot)") {
     Component(tagTreeLoader, "TagTreeImporter", "Spring Component", "Upserts the tag hierarchy from canonical-tag-tree.xlsx via TagService (by canonical tag_path).")
     Component(personRegLoader, "PersonRegisterImporter", "Spring Component", "Upserts register persons from canonical-persons.xlsx via PersonService (by normalizer person_id).")
     Component(personTreeLoader, "PersonTreeImporter", "Spring Component", "Upserts tree persons + relationships from canonical-persons-tree.json via PersonService and RelationshipService.")
-    Component(docLoader, "DocumentImporter", "Spring Component", "Loads canonical-documents.xlsx: routes attribution register-first (raw cell always retained in sender_text/receiver_text), parses clean dates, builds an honest precision-aware title via DocumentTitleFormatter, keeps the S3 upload + thumbnail plumbing, and ports the path-traversal / homoglyph / absolute-path / %PDF magic-byte security guards.")
+    Component(docLoader, "DocumentImporter", "Spring Component", "Loads canonical-documents.xlsx: routes attribution register-first (raw cell always retained in sender_text/receiver_text), parses clean dates, builds an honest precision-aware title via DocumentTitleFormatter, keeps the S3 upload + thumbnail plumbing, and resolves each PDF by index (importDir/<index>.pdf) guarded by strict index validation + canonical-path containment + %PDF magic-byte check (no recursive walk).")
     Component(titleFmt, "DocumentTitleFormatter", "Pure helper", "Formats the date label baked into an import title at exactly the data's precision (MONTH -> 'Juni 1916', never a fabricated day). Mirrors the frontend formatDocumentDate; both are pinned to docs/date-label-fixtures.json (#666).")
     Component(sheetReader, "CanonicalSheetReader", "POI helper", "Maps a canonical .xlsx by header name (no positional indices), splits pipe-delimited list columns, fails closed (IMPORT_ARTIFACT_INVALID) on a missing required header.")
     Component(minioConf, "MinioConfig", "Spring @Configuration", "Creates the S3Client and S3Presigner beans with path-style access for MinIO. Validates MinIO connectivity on startup.")