feat: Kalender-Sharing, Gruppen, iCal Import/Export & Ersteller (Server)

Kollaborations-Features ausschliesslich fuer lokale Kalender:

- Sharing: calendar_shares-Tabelle, GET/POST/DELETE /api/local/calendars/{id}/shares
  (nur Besitzer), GET /api/users/directory, geteilte Kalender in
  GET /api/local/calendars (shared_by/permission/owned) und im Merge-Read.
- Gruppen: groups/group_members/group_calendars + /api/groups-Router inkl.
  kombinierter Ansicht /api/groups/{id}/combined (owner + is_group_event).
- Ersteller: local_events.creator_id (serverseitig gesetzt) + creator_name_external
  aus ORGANIZER; creator-Feld in allen lokalen Event-Responses.
- Private-Flag: local_events.is_private + user_settings.private_event_visibility
  (hidden|busy), Filterung in der Gruppenansicht.
- iCal Import/Export: ical_io.py, POST /api/local/calendars/{id}/import,
  POST /api/local/import, GET /api/local/calendars/{id}/export.
- Zentraler Berechtigungs-Helper (permissions.py) und gemeinsamer Event-Dict-
  Builder (local_events_util.py) ersetzen die Nur-Besitzer-Filter.
- pytest-Suite (12 Tests) fuer Sharing, Gruppen, Parser, Private-Filterung.

Additiv & rueckwaertskompatibel; Migrationen in main.py._migrate().

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

backend/routers/users_router.py

@@ -35,6 +35,25 @@ def list_users(
     return [_user_dict(u) for u in db.query(models.User).all()]
 
 
+@router.get("/directory")
+def user_directory(
+    db: Session = Depends(get_db),
+    current_user: models.User = Depends(get_current_user),
+):
+    """Lightweight list of all users (id + display_name) for sharing/group pickers.
+
+    Available to any authenticated user (unlike GET / which is admin-only).
+    Excludes the requesting user.
+    """
+    users = (
+        db.query(models.User)
+        .filter(models.User.id != current_user.id)
+        .order_by(models.User.username)
+        .all()
+    )
+    return [{"id": u.id, "display_name": u.username} for u in users]
+
+
 @router.post("/")
 def create_user(
     req: CreateUserRequest,