List the flow templates available for the current brand

Returns the ids of the flow templates that may be used on the current brand — the one given by blogId, or the session's current brand when it is omitted. A template restricted by its own feature flag is listed only when both the brand owner and the caller qualify for it, so a user with a shared role on the brand may get a smaller catalogue than its owner; for an agency session the agency owner answers for both. Templates with no flag are listed, except those with no implementation yet. The subscription plan checked is the brand owner's.