Performance Optimierungen: Rate Limit, created_at Index, Keyset-Pagination

1. Rate Limit erhoeht: 60 -> 300 Requests/Minute (fuer 100+ User)
2. Migration 0099: created_at DESC Index auf allen Tabellen (Order by Performance)
3. Keyset-Pagination: optionaler cursor Parameter fuer contacts API
   - cursor=UUID nutzt WHERE id > cursor statt OFFSET
   - Backward compatible: ohne cursor wird page/page_size genutzt
   - next_cursor in Response fuer naechste Seite

Tests: 43/43 bestanden

app/routes/contacts.py

@@ -65,10 +65,14 @@ async def list_contacts(
     folder_id: str | None = Query(None),
     sort_by: str = Query("displayname"),
     sort_order: str = Query("asc", pattern="^(asc|desc)$"),
+    cursor: str | None = Query(None, description="Keyset pagination cursor (contact UUID)"),
     db: AsyncSession = Depends(get_db),
     current_user: dict = Depends(require_permission("contacts:read")),
 ):
-    """List contacts with pagination, FTS search, type/folder filter, sorting."""
+    """List contacts with pagination, FTS search, type/folder filter, sorting.
+
+    Supports keyset pagination via ``cursor`` parameter for large datasets.
+    """
     tenant_id = uuid.UUID(current_user["tenant_id"])
     user_id = uuid.UUID(current_user["user_id"])
     is_admin = current_user.get("is_system_admin", False)
@@ -80,6 +84,7 @@ async def list_contacts(
         resolved_perms=current_user,
         user_id=user_id,
         is_system_admin=is_admin,
+        cursor=cursor,
     )