SQLite timestamp columns have incorrect types

The two columns `run_timestamp` and `completed_timestamp` in the `runs` table of the SQLite database have type `INTEGER`, but are in fact used to store floats.


I am not really sure how or why that even works in QCoDeS, in particular, which converter is used to read back these numbers? Why not `int`?

In any case, that typing becomes a problem when third-party applications try to parse the DB file, since these applications will most likely "Believe the type". In the best case, doing so leads to the timestamps losing their sub-second precision, will can cause hard-to-debug problems down the line, since identical datasets will have slightly different timestamps. In the worst case, the timestamps simply appear to be invalid.

I propose that we make a type converting DB upgrade, changing these two `INTEGER` types to `REAL`. Alternatively, we could use an integer number of milliseconds, but that is WAY more work and seems like the wrong solution to me.

@QCoDeS/core

Start by locating the SQLite schema for the runs table and the database upgrade or migration path that handles existing files. Confirm how run_timestamp and completed_timestamp are written and read, then ensure the schema reports REAL without losing existing sub-second precision; done means new and upgraded databases expose both columns with the correct type.

Written by the indexing model from the issue text.