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-frontend-3d-administration.puml

@@ -12,7 +12,7 @@ System_Boundary(frontend, "Web Frontend (SvelteKit / SSR)") {
     Component(adminGroups, "/admin/groups, /admin/groups/[id], /admin/groups/new", "SvelteKit Routes", "Permission group management: create/edit groups and their permission sets.")
     Component(adminTags, "/admin/tags and /admin/tags/[id]", "SvelteKit Routes", "Tag administration: edit tag hierarchy, merge tags, delete subtrees.")
     Component(adminOcr, "/admin/ocr and /admin/ocr/[personId]", "SvelteKit Routes", "Global and per-person OCR configuration. Manages script types and triggers sender model training.")
-    Component(adminSystem, "/admin/system", "SvelteKit Route", "System status panel. Triggers Excel/ODS mass import (POST /api/admin/trigger-import). Displays import state.")
+    Component(adminSystem, "/admin/system", "SvelteKit Route", "System status panel. Triggers the canonical import (POST /api/admin/trigger-import). Displays import state.")
     Component(hilfe, "/hilfe/transkription", "SvelteKit Route", "Static transcription style guide for Kurrent and Sütterlin character recognition. No backend calls.")
 }