Standardise PostgreSQL "current" timestamp usage

PostgreSQL offers various functions and variables for accessing the current date/time: https://www.postgresql.org/docs/current/functions-datetime.html#FUNCTIONS-DATETIME-CURRENT

In the codebase, 4 variants are used:

```sh
echo &&
echo " count variant" &&
git grep -E '(localtime\w*)|current_date|(current_time\w*)|(\bnow\(\))|(timeofday\(\))|(\w+_timestamp\(\))' |
grep -Fv 'Date.now()' |
grep -Eo '(localtime\w*)|current_date|(current_time\w*)|(\bnow\(\))|(timeofday\(\))|(\w+_timestamp\(\))' |
sort |
uniq --count

count variant
40 clock_timestamp()
5 current_date
2 current_timestamp
42 now()
```

It would be helpful to understand which variant(s) are preferred by default, and why alternatives are used in specific situations.