Shadow string timestamps with epoch seconds in database

[matthiasemde]

Since SQLite does not support a native date format, we use ISO encoded strings to save timestamps in the following format:

2024-03-17T14:00:01.053+01:00[Europe/Berlin]

This has multiple drawbacks:

1. since the datatime() function of SQLite only understands 2024-03-17T14:00:01.053+01:00 but not the [TZone] suffix, we have to manually strip this suffix away on every query before feeding the ISO-string into datetime().

2. Database queries that compare timestamps first need to convert these strings back to datetime using datetime which costs a lot of time. If we shadow the string encoded timestamps with epoch seconds and use these internally for comparisons, we can save a lot of computation.