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/TODO-backend.md

@@ -94,17 +94,6 @@ The schema includes `spring_session` and `spring_session_attributes` tables, but
 
 ---
 
-### `MassImportService` provides no status or error feedback
-**File:** `service/MassImportService.java`, `controller/AdminController.java`
-
-`/api/admin/trigger-import` returns immediately (async), but there is no way for the admin to know whether the import succeeded, failed, or is still running. Errors during async execution are silently swallowed.
-
-**Fix options:**
-- Store import job status in a DB table (`import_jobs`) with state (`RUNNING`, `DONE`, `FAILED`) and expose a `GET /api/admin/import-status` endpoint
-- Alternatively, make the endpoint synchronous since it already blocks on file I/O — only use async if you need true non-blocking behaviour
-
----
-
 ## Missing Capabilities
 
 ### No test coverage
@@ -114,7 +103,7 @@ The only test is a Spring context load test. No unit or integration tests exist
 
 **Suggested starting points (highest value for effort):**
 1. `DocumentSpecifications` — pure logic, easy to unit test with an in-memory H2 or Testcontainers PostgreSQL
-2. `ExcelService` — parsing logic, test with fixture `.xlsx` files (one exists in `api_tests/`)
+2. Canonical import loaders (`CanonicalSheetReader`, `DocumentImporter`, etc.) — parsing/upsert logic, test with fixture canonical `.xlsx` files
 3. `PermissionAspect` — security logic should be tested; use `@WithMockUser` from Spring Security Test
 
 ---