fix(collections): the create modal's seed POST 400'd — echo cannot bind a form string into an any-typed field

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).

internal/handlers/collections.go

@@ -46,8 +46,16 @@ type CreateCollectionRequest struct {
 	// smart membership runs through the qf engine. INVALID trees are a
 	// 400 (ParseFilterTree at the handler); a tree supersedes
 	// auto_assign_rules. Persisted as-is (any). RawMessage for
-	// round-trip safety.
-	FilterTree any `form:"filter_tree" json:"filter_tree"`
+	// round-trip safety. JSON bodies bind FilterTree; FORM bodies
+	// (the create modal's urlencoded hx-post) bind FilterTreeForm —
+	// echo's binder cannot set an any-typed field from a form string
+	// (the create-modal seed POST 400'd "invalid request" on it,
+	// 2026-10-31).
+	FilterTree any `json:"filter_tree"`
+	// FilterTreeForm mirrors filter_tree for application/x-www-form-
+	// urlencoded submissions: the raw JSON as a string, validated by
+	// the same parseRequestFilterTree path.
+	FilterTreeForm string `form:"filter_tree"`
 }
 
 // treeFilterJSON returns the request's filter tree as JSON storage
@@ -85,6 +93,16 @@ func parseRequestFilterTree(payload any) ([]byte, error) {
 	return b, nil
 }
 
+// resolveTreePayload — the tree payload from either transport: JSON
+// bodies carry the object in FilterTree, form bodies (the create
+// modal's urlencoded hx-post) the raw JSON string in FilterTreeForm.
+func resolveTreePayload(jsonTree any, formTree string) any {
+	if formTree != "" {
+		return formTree
+	}
+	return jsonTree
+}
+
 type UpdateCollectionRequest struct {
 	Name string `form:"name" json:"name" validate:"required"`
 	// Smart-collection match semantics (item 2.4): "any" (OR) | "all"
@@ -93,8 +111,11 @@ type UpdateCollectionRequest struct {
 	// The unified filter tree (item 3.2): when present the collection's
 	// smart membership runs through the qf engine. INVALID trees are a
 	// 400 (ParseFilterTree at the handler); a tree supersedes
-	// auto_assign_rules.
-	FilterTree      any                    `form:"filter_tree" json:"filter_tree"`
+	// auto_assign_rules. JSON bodies bind FilterTree; form bodies bind
+	// FilterTreeForm (echo's binder cannot set an any-typed field from
+	// a form string — see CreateCollectionRequest).
+	FilterTree      any                    `json:"filter_tree"`
+	FilterTreeForm  string                 `form:"filter_tree"`
 	Description     string                 `form:"description" json:"description"`
 	Color           string                 `form:"color" json:"color"`
 	Icon            string                 `form:"icon" json:"icon"`
@@ -186,7 +207,7 @@ func (h *CollectionHandler) CreateCollection(c *echo.Context) error {
 		return c.JSON(http.StatusBadRequest, map[string]string{"error": err.Error()})
 	}
 
-	treeJSON, treeErr := parseRequestFilterTree(req.FilterTree)
+	treeJSON, treeErr := parseRequestFilterTree(resolveTreePayload(req.FilterTree, req.FilterTreeForm))
 	if treeErr != nil {
 		return c.JSON(http.StatusBadRequest, map[string]string{"error": treeErr.Error()})
 	}
@@ -507,7 +528,7 @@ func (h *CollectionHandler) UpdateCollection(c *echo.Context) error {
 		req.ViewSettings["show_up_next"] = *v
 	}
 
-	treeJSON, treeErr := parseRequestFilterTree(req.FilterTree)
+	treeJSON, treeErr := parseRequestFilterTree(resolveTreePayload(req.FilterTree, req.FilterTreeForm))
 	if treeErr != nil {
 		return c.JSON(http.StatusBadRequest, map[string]string{"error": treeErr.Error()})
 	}