Persist chat history

priv/repo/migrations/20260317130441_create_chats.exs

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+defmodule MusicLibrary.Repo.Migrations.CreateChats do
+  use Ecto.Migration
+
+  def change do
+    create table(:chats, primary_key: false) do
+      add :id, :binary_id, primary_key: true
+      add :entity, :string, null: false
+      add :musicbrainz_id, :uuid, null: false
+      add :topic, :string
+
+      timestamps(type: :utc_datetime)
+    end
+
+    # Listing chats for a specific entity
+    create index(:chats, [:entity, :musicbrainz_id])
+
+    # Ordered listing of chats for a specific entity
+    create index(:chats, [:entity, :musicbrainz_id, :updated_at])
+
+    create table(:chat_messages, primary_key: false) do
+      add :id, :binary_id, primary_key: true
+      add :role, :string, null: false
+      add :content, :text, null: false
+      add :position, :integer, null: false
+
+      add :chat_id, references(:chats, type: :binary_id, on_delete: :delete_all), null: false
+
+      timestamps(type: :utc_datetime)
+    end
+
+    # Ordered listing of messages within a chat
+    create index(:chat_messages, [:chat_id, :position])
+  end
+end