Having invested hundreds of millions in mounting a series version of “The Lord of the Rings,” Amazon has gotten its money’s worth in production values but not storytelling, with a handsome prequel that could leave all but the most devoted Hobbits feeling more bored than lord. “The Rings of Power” should be a source of considerable curiosity, but amid an onslaught of big-budget streaming fare, this seems undeserving to rule them all.