fix(models): correct inverted belongsTo keys on ChatMessage.session (#921)

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.

admin/app/models/chat_message.ts

@@ -18,7 +18,7 @@ export default class ChatMessage extends BaseModel {
   @column()
   declare content: string
 
-  @belongsTo(() => ChatSession, { foreignKey: 'id', localKey: 'session_id' })
+  @belongsTo(() => ChatSession, { foreignKey: 'session_id', localKey: 'id' })
   declare session: BelongsTo<typeof ChatSession>
 
   @column.dateTime({ autoCreate: true })