Replace awesome_nested_set with a plain parent_id-based NestedTree concern

Root-caused this session: appending a child to any node never widened
that parent's own rgt boundary, on the pinned revision (Gemfile tracked
main directly, chasing a too-conservative gemspec constraint - not, as
first assumed, a deliberate pin to avoid a known bug). Reproduced
cleanly on a single ordinary create with no concurrency and no bulk
operation involved, confirmed via the gem's own SetValidator, then
confirmed as the root cause of nodes_controller_test.rb's 3 long-standing
"pre-existing" failures - not three separate mysteries, one bug.

admin_controller's sitemap needed its own real conversion, not just a
drop-in: awesome_nested_set's lft column implicitly provided correct
depth-first tree order for free, which the old code combined with a
separate class-level each_with_level iterator. Both replaced by one
method, self_and_descendants_ordered_with_level, computing an ordered
[node, level] list in a single query-then-walk pass - checked against
the actual view template first (admin/index.html.erb) rather than
assumed, since it relies on list order alone to render correct visual
nesting.

lft/rgt/depth columns intentionally left in schema, unused - dropping
them is a separate, deliberately deferred migration once this is proven
running for a while, not bundled with the behavior change.

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