greader: `crawlTimeMsec` is sent at microsecond precision, instead of conventional millisecond precision

[mbestavros]

I'm contributing to the Read You Android app, and we've run into a strange bug that appears to be Miniflux-related. The full writeup is here, but the gist is that Miniflux sends the crawlTimeMsec value provided as part of the contentItem schema at microsecond precision instead of millisecond precision.

This seems problematic for two reasons:

• The parameter itself - crawlTimeMsec (emphasis mine) - suggests it should be millisecond precision.
• More importantly, other GReader API implementations send this parameter at millisecond precision. Notably, FreshRSS' API sends crawlTimeMsec and timestampUsec at different precisions. FeedHQ, though defunct, also used milliseconds.

Miniflux should consider replicating the FreshRSS functionality so clients do not have to tailor fixes for Miniflux specifically.