feat: add buildGroupSchedulePattern helper

[wsparcie]

• detect weekly, biweekly, weekly_with_exceptions, irregular patterns
• calculate parity: even, odd, all, unknown
• detect exceptions and gaps in schedule
• handle edge cases: single occurrence, irregular gaps, holiday breaks

[qamarq]

po pierwsze endpoint to :
tt/classgroup_dates2

po drugie takie wez pola: start_time|end_time|name|room_number|building_name|classtype_id

po trzecie tu masz przykladowy czesc z arrayki:

{
    "start_time": "2026-03-03 13:15:00",
    "end_time": "2026-03-03 15:00:00",
    "name": {
        "pl": "Zajęcia laboratoryjne",
        "en": "Laboratory"
    },
    "room_number": "107b",
    "building_name": {
        "pl": "Budynek dydaktyczny [D-2]",
        "en": ""
    },
    "classtype_id": "L"
},