Add created_at and last_updated_at timestamps

[G8XSU]

We use application level timestamps to maintain
dialect compatibility between different SQL variations. We truncate timestamp to granularity of days for privacy reasons.

[jkczyz]

Change looks good, but I have a couple questions:

• By "dialect compatibility" do you mean implementations use say seconds vs milliseconds, so we need to normalize on something with common granularity?
• What are the new columns used for?

[G8XSU]

"dialect compatibility"

There are other ways to achieve created_at and updated_at timestamps with only using SQL query or using DB procedures etc. But we do this at application level, to keep sql query simple(having this truncate logic and procedures in sql is complex) and for dialect compatibility, which means this same query will more or less work b/w postgres/mysql/sqlite etc. whereas for other methods we might have to rely on exact db sql syntax.

What are the new columns used for?

They aren't used for any specific rn, mainly useful for audit purpose. Could be used by hosting providers to "expire data" if lets say a user hasn't paid in last 2 years.
It might also be useful if spiral wants to host a test-vss-instance for developers to try out, but doesn't want to store data forever. (for e.g. we can delete data older than ~2 weeks.)