The first real-browser create of an Episodes-scope collection failed
with "invalid request": the modal's hx-post submits
application/x-www-form-urlencoded, and the form value for filter_tree
had to bind into CreateCollectionRequest.FilterTree (any) — echo's
form binder errors on any-typed fields instead of setting the string,
so every form carrying a tree 400'd before validation even ran. JSON
bodies (the save-filter bridge, the editor's Apply) were unaffected,
which is why no API-level or Go test caught it: only a browser posting
the real form shape exercised the transport.
Fix: the form variant binds a dedicated string field
(FilterTreeForm, form:"filter_tree") on both request structs while
FilterTree keeps the JSON tag; resolveTreePayload picks whichever
transport arrived and both ride the same parseRequestFilterTree
validation. Pinned by TestEpisodeCollectionsFormSeedCreate (form-encoded
seed → 201 → scope round-trips).