docs: update SQLite timestamp_ms example to allow millisecond defaults

[woobs]

The current SQLite examples for setting the default value for a timestamp_ms column do not include the actual millisecond value of the current time, simply multiplying the UNIX timestamp in seconds by 1000. We can, however, obtain the millisecond value with the 'subsec' modifier to the unixepoch function, which returns a float, which we can cast to an integer value:

select unixepoch() * 1000, unixepoch('subsec'), CAST(unixepoch('subsec') * 1000 as INT);

unixepoch() * 1000	unixepoch('subsec')	CAST(unixepoch('subsec') * 1000 as INT)
1733502310000	1733502310.844	1733502310844

One concern is this note from the SQLite Date/Time Function Docs:

The current implemention [sic] increases the resolution from seconds to milliseconds, but this might increase to a higher resolution in future releases of SQLite

But I don't think that would change the output.