[nexus] use raw query builder for sitrep version insertion CTE (#9380)

Depends on #9335.

In [this comment][1] on PR #9335, @smklein suggested using the
`raw_query_builder` API for writing CTEs, rather than using Diesel's
`QueryFragment` API. This turns out to be a lot less unpleasant and,
despite performing less validation of the generated SQL, actually is a
lot *less* likely to introduce hard-to-debug SQL syntax errors. Since
most of the query can be written in a single string literal, you
actually see more or less what you're gonna get, which is much less
obvious with Diesel's API. The `InsertSitrepVersionQuery` CTE in the
same module still uses the `QueryFragment` API, rather than the
`raw_query_builder` module, since I didn't want to change the existing
code in that branch.

This PR refactors the CTE for inserting a new sitrep version to also use
`raw_query_builder`, which is much nicer and should be easier to modify
in the future. There should be no functional changes here, just a change
to use a different API to generate the query.

Incidentally, this was one of the first times I had real success getting
Claude to do a fairly trivial "transform this code from using one API to
using another" task --- it wrote most of the first commit, and I tweaked
some of its style decisions I didn't love.

[1]:
#9335 (comment)

Author: hawkw