fix(search): keep the empty-qf 400 at the wire

The empty-tree grammar relaxation (create-modal episodes seed) must not
leak into the search deep-link contract: qf={"mode":"all","children":[]}
as a QUERY would silently mean "match everything". The seed's meaning —
a rules-less collection matching nothing — lives in the stored tree and
its composers; the query entry point now rejects a rule-less tree
explicitly, preserving the pinned 400 that TestFilterQFValidationAndPrecedence
guards (the test caught the leak during this round's full-suite run).

internal/handlers/media.go

@@ -2559,6 +2559,13 @@ func ParseMediaSearchParams(c searchParamSource) services.SearchParams {
 		tree, err := services.ParseFilterTree(qf)
 		if err != nil {
 			params.FilterParseErr = err.Error()
+		} else if len(tree.Children) == 0 {
+			// A qf with no rules is not a browse query: the deep-link
+			// contract answers 400 (pinned). The empty root gained
+			// meaning only as a STORED collection seed — where the
+			// composers define it as matching nothing. As a query it
+			// would silently mean "everything", so reject at the wire.
+			params.FilterParseErr = "qf tree has no rules"
 		} else {
 			params.FilterTree = tree
 		}