perf(calendar): pre-filter ICS text before parsing

[KristjanESPERANTO]

This adds a pre-filter before parsing calendar ICS data. It drops simple one-off events that are clearly outside the configured date range, so node-ical has less work to do on large calendars.

Recurring events, overrides, RDATE, and unclear cases are kept and still handled by the normal parser logic.

I’ve spent quite a bit of time tightening this up because calendar data can be tricky. The pre-filter is intentionally conservative: when it is not sure, it keeps the event and lets the existing parser handle it.

Why? Some calendars contain years of old non-recurring events. Parsing all of them is unnecessary when MagicMirror only needs the configured visible window.

Doubts: The whole thing is a bit complex, and I'm wondering whether it makes sense to merge it. There have been hardly any complaints, and sooner or later, we won't be supporting slow Raspberry Pis anymore anyway (see #3796). Is it worth increasing the complexity of our codebase?

Solves #4103.

[rejas]

Just a thought: would a library like https://github.com/runely/ics-filter offload some complexity from this PR?

[sdetweil]

@rejas that filters out ALL past events (we want SOME, sometimes)

the filter will only filter out past events

[sdetweil]

i also replied in the issue, not in the PR.

[KristjanESPERANTO]

Thanks @rejas for the pointer to ics-filter! I had a closer look and did some tests - I think the general approach of that package fits well, though there are a few edges. I just opened a first PR there to address one of them (case-insensitive property names): runely/ics-filter#1

I'm setting this to draft for now and waiting for the author to respond. I'd rather improve an existing package than maintain our own implementation here :)

[KristjanESPERANTO]

that filters out ALL past events (we want SOME, sometimes)

We can pass now - maximumNumberOfDays as the min date to icsFilter(), which is the same window filterEvents() (in ./defaultmodules/calendar/calendarfetcherutils.js) already uses for past events.