feat(C.5): Modularer Import/Export — Shared Helpers, Preview/Mapping, Background Jobs, Partial-Failure

C5-BASE: app/services/import_export_helpers.py (NEU, 352 Zeilen)
- parse_csv/json/xlsx, write_csv/json/xlsx, map_fields, suggest_mapping
- validate_row, build_error_report, build_import_result, detect_format

C5-PREVIEW: POST /import/preview + POST /import/validate
- Preview gibt erste 10 Zeilen + Spalten + Mapping-Vorschlag
- Validate gibt Fehler-Report ohne Import

C5-JOB: app/services/import_export_jobs.py (NEU, 165 Zeilen)
- ARQ Background Job für Files >1000 Zeilen
- Job-Status: pending/processing/completed/partial_success/failed
- GET /import/status/{job_id} — Status + Progress + Fehler-Report
- Partial-Failure: try/except pro Zeile, fehlerhafte gesammelt, erfolgreiche committet

C5-CONTACT+C5-COMPANY: Handler auf Shared Helpers umgestellt
C5-UI: ImportWizard.tsx (5 Steps: Upload→Preview→Validation→Review→Result)
C5-TEST: 45 Tests in test_import_export.py — alle grün
C5-DOC: Plugin-Dev-Guide Kapitel 31 (Import/Export Handler)

app/core/worker.py

@@ -225,6 +225,7 @@ def _lazy_register_plugin_jobs() -> None:
         "app.plugins.builtins.automation.agent_runner",
         "app.plugins.builtins.automation.execution_engine",
         "app.plugins.builtins.tasks.jobs",
+        "app.services.import_export_jobs",
     ]
     for mod_name in plugin_job_modules:
         try: