foreignKey/localKey were swapped on the ChatMessage → ChatSession
relation. Per Lucid's belongsTo contract, foreignKey is the column
on the child model and localKey is on the parent — so this must be
{ foreignKey: 'session_id', localKey: 'id' }, mirroring the inverse
hasMany on ChatSession.
The relation is not currently preloaded anywhere in the codebase, so
no runtime behavior changes today; this closes a latent bug that
would have broken any future preload('session') call.
