Computes field-level diffs from Ecto changesets.

Associations (has_many/has_one/belongs_to/many_to_many) and embeds (embeds_one/embeds_many) are represented by Ecto as nested %Ecto.Changeset{} structs (or lists of them) inside the parent changeset's changes. Those are diffed recursively instead of being dumped as an opaque struct, so a field-level before/after is captured for the parent and anything nested under it.

A nested schema can opt into its own field-level filtering by adding use AuditTrail.Schema, track: [...] to the embedded/associated schema module itself — the parent's track: list only ever filtered top-level fields, so this is what makes track: meaningful for what's inside an embed/assoc. Nested schemas that don't declare it keep the old behavior (every nested field included once the parent field is tracked).