feat: sync calendar instantly on changes

[st3iny]

This approach requires notify_push.

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 0% with 31 lines in your changes missing coverage. Please review.

Project coverage is 29.18%. Comparing base (bd73f27) to head (97948db). Report is 9 commits behind head on main.

Files with missing lines	Patch %	Lines
src/store/calendars.js	0.00%	16 Missing and 2 partials :warning:
src/services/notifyService.js	0.00%	10 Missing and 1 partial :warning:
src/views/Calendar.vue	0.00%	2 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #6364 +/- ## ============================================ + Coverage 23.54% 29.18% +5.63% - Complexity 454 916 +462 ============================================ Files 247 291 +44 Lines 11700 13988 +2288 Branches 2203 2204 +1 ============================================ + Hits 2755 4082 +1327 - Misses 8625 9583 +958 - Partials 320 323 +3 ``` | [Flag](https://app.codecov.io/gh/nextcloud/calendar/pull/6364/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=nextcloud) | Coverage Δ | | |---|---|---| | [javascript](https://app.codecov.io/gh/nextcloud/calendar/pull/6364/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=nextcloud) | `15.05% <0.00%> (-0.03%)` | :arrow_down: | | [php](https://app.codecov.io/gh/nextcloud/calendar/pull/6364/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=nextcloud) | `58.97% <ø> (-0.56%)` | :arrow_down: | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=nextcloud#carryforward-flags-in-the-pull-request-comment) to find out more.

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[tcitworld]

This is nice, but I feel working with Sabre's DAV Push to use WebPush would be half the work and bring the same functionality to 3rd-party clients for the same cost?