SERAC activities filtering does not work

filtering xreports on activities returns no result : https://www.camptocamp.org/xreports?act=skitouring
maybe linked with the reorganization of activities dedicated to SERAC (https://github.com/c2corg/v6_api/pull/761/commits/c0f7e6011b11207501e2106f5f6cb2c2abd88d87 ?)

Reproduce the xreports activity filter at the linked Camptocamp URL, then inspect the SERAC activity reorganization referenced in pull request 761, commit c0f7e6011b11207501e2106f5f6cb2c2abd88d87. Trace how activity filtering is handled for xreports and confirm that skitouring returns matching results; add or update coverage if an existing test entry point is found.

Written by the indexing model from the issue text.