feat(#359): W4a Phase 1 — Import/Export Contribution-Architektur Kern

- Format-Registry (app/core/importexport_registry.py): FormatHandler-Protokoll,
  available_for() Schnittmenge, Singleton + Testing-Reset
- importexport_formats-Plugin (csv/json/xlsx), lifecycle-korrekt: on_activate
  registriert Handler in der Core-Registry, on_deactivate unregistriert
- ContactsContract: importexport-Beitrag (ie_*-Methoden) — Contacts besitzt
  seine Import/Export-Fachlogik jetzt selbst
- import_export_service.py: generische Engine, delegiert generisch ueber
  registry.list_discovered() an den besitzenden Contract (keine hartcodierten
  Plugin-Namen mehr); Signaturen identisch
- Funktionserhalt bewiesen: 45/45 import_export-Suite passed (inkl.
  Fehler-Multiplizitaet: 2 failed rows -> 3 total_errors, erreicht via
  ie_required-Weitergabe + ie_row_valid-Nur-contacts-Early-Return)

fixes #359 (Phase 1)

app/plugins/builtins/importexport_formats/plugin.py

@@ -0,0 +1,52 @@
+"""Import/export formats plugin — provides standard file formats as plugins.
+
+Part of the W4a contribution architecture (Spec #359): file formats live in
+this plugin instead of being hardcoded in the core orchestrator. New formats
+(e.g. PDF later) are added as additional format plugins without touching the
+core.
+
+Lifecycle: registers the bundled handlers (csv, json, xlsx) in the core
+format registry on activate, unregisters them on deactivate.
+"""
+
+from __future__ import annotations
+
+from typing import Any
+
+from app.core.importexport_registry import get_format_registry
+from app.plugins.base import BasePlugin
+from app.plugins.builtins.importexport_formats.formats import CsvFormat, JsonFormat, XlsxFormat
+from app.plugins.manifest import PluginManifest
+
+
+class ImportExportFormatsPlugin(BasePlugin):
+    """Bundles the standard import/export file formats."""
+
+    manifest = PluginManifest(
+        name="importexport_formats",
+        version="1.0.0",
+        display_name="Import/Export Formate",
+        description="Standard-Formate für Import/Export: CSV, JSON, XLSX.",
+        dependencies=[],
+        routes=[],
+        events=[],
+        migrations=[],
+        permissions=[],
+        is_core=True,
+    )
+
+    def __init__(self) -> None:
+        super().__init__()
+        self._handlers: list[Any] = [CsvFormat(), JsonFormat(), XlsxFormat()]
+
+    async def on_activate(self, db, service_container, event_bus) -> None:
+        await super().on_activate(db, service_container, event_bus)
+        registry = get_format_registry()
+        for handler in self._handlers:
+            registry.register(handler)
+
+    async def on_deactivate(self, db, service_container, event_bus) -> None:
+        await super().on_deactivate(db, service_container, event_bus)
+        registry = get_format_registry()
+        for handler in self._handlers:
+            registry.unregister(handler.format_id)