Upgrade or replace ics library (incompatible with tatsu 5.8+)

## Problem

`ics` 0.7.2 (released 2022-07-06) uses `tatsu` internally and passes a `buffer_class` keyword argument to `tatsu`'s `ParserConfig`. This parameter was removed in `tatsu` 5.8, causing a `ValueError` at import time:

```
ValueError: Unknown settings for ParserConfig: {'buffer_class'}
```

## Current workaround

`tatsu` is pinned to `<5.8` in `pyproject.toml` to keep `ics` working. This blocks upgrading `tatsu` and may conflict with other dependencies over time.

## Desired outcome

Either:
1. **Upgrade `ics`** — if a newer release fixes tatsu compatibility (as of 2026-03-21, the latest stable is still 0.7.2)
2. **Replace `ics`** with a maintained alternative (e.g. `icalendar`) if `ics` remains unmaintained

## References

- `ics` PyPI: https://pypi.org/project/ics/
- `tatsu` changelog: https://tatsu.readthedocs.io/en/stable/CHANGELOG.html