feat: closing last gaps in backend functions we have available

lib/bds/mcp/proposal.ex

@@ -0,0 +1,24 @@
+defmodule BDS.MCP.Proposal do
+  @moduledoc false
+
+  use Ecto.Schema
+  import Ecto.Changeset
+
+  @primary_key {:id, :string, autogenerate: false}
+
+  schema "mcp_proposals" do
+    field :kind, :string
+    field :status, Ecto.Enum, values: [:pending, :accepted, :discarded, :expired], default: :pending
+    field :entity_id, :string
+    field :data, :map
+    field :created_at, :integer
+    field :expires_at, :integer
+  end
+
+  def changeset(proposal, attrs) do
+    proposal
+    |> cast(attrs, [:id, :kind, :status, :entity_id, :data, :created_at, :expires_at], empty_values: [nil])
+    |> validate_required([:id, :kind, :status, :entity_id, :data, :created_at, :expires_at])
+    |> unique_constraint(:status, name: :mcp_proposals_entity_idx)
+  end
+end