Add RelatedAssetsController: search, attach, detach, reorder

Backend for the asset-picker rebuild -- replaces the plan to dump
every image asset in the system into a hidden, unfiltered browse
panel on every node edit (the actual current behavior, confirmed by
reading nodes/edit.html.erb directly) with a small, name-scoped search
endpoint plus create/destroy/update for attach, detach, and reorder.

No schema change needed -- RelatedAsset already had everything this
requires (asset_id, page_id, an acts_as_list position). search excludes
assets already attached to the page, keeping results meaningfully small
given hundreds of assets total but only a handful per node in practice.
create is find_or_create_by! rather than a bare create!, guarding
against the same asset being attached twice from two separate search
results. update leans on acts_as_list's own insert_at rather than
custom position-shifting logic.

Node#editable_page (autosave || draft || head) is extracted since this
is now its third call site with identical logic -- deliberately not
touching nodes#show, which resolves draft || head without autosave on
purpose, a different and correct semantic for "current committed
state" versus "what's actively being edited."

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