fix: upgrade caldav for icalendar 7.x compatibility (#397)

## Summary
- Upgrade `caldav` dependency from `==1.3.6` to `>=2.2.6` to fix
incompatibility with `icalendar>=7.0.0`
- `caldav` 1.3.6 accesses `icalendar.cal.component_factory` as a dict,
but icalendar 7.x refactored it into a module, causing `TypeError:
'module' object is not subscriptable` in all calendar operations

## Test plan
- [x] Calendar tests pass locally (2/2 passed with both client and app
modes)
- [x] Full test suite passes: **588 passed, 0 failed, 14 skipped**
- [x] Verified `caldav` 2.x API is backwards compatible — `DAVClient`
and `DAVResponse` imports and constructor unchanged in
`nc_py_api/calendar_api.py`

Author: bigcat88

pyproject.toml

@@ -61,7 +61,7 @@ optional-dependencies.bench = [
   "py-cpuinfo",
 ]
 optional-dependencies.calendar = [
-  "caldav==1.3.6",
+  "caldav>=2.2.6",
 ]
 optional-dependencies.dev = [
   "nc-py-api[bench,calendar,dev-min]",