feat: accept calibration readings only from a chosen tag

lib/localiser/web/controllers/sensor_controller.ex

@@ -116,6 +116,16 @@ defmodule Localiser.Web.Controllers.SensorController do
       unauthorized: {"Unauthorized", "application/json", Schemas.Error}
     ]
 
+  operation :calibration_set_tag,
+    summary: "Commit to a specific tag for calibration stages",
+    parameters: [id: [in: :path, type: :integer, required: true]],
+    request_body: {"Tag params", "application/json", Schemas.CalibrationTagParams, required: true},
+    responses: [
+      ok: {"Tag committed", "application/json", Schemas.CalibrationBeginResponse},
+      unprocessable_entity: {"Stage active or not in calibration mode", "application/json", Schemas.Error},
+      unauthorized: {"Unauthorized", "application/json", Schemas.Error}
+    ]
+
   operation :calibration_stage_start,
     summary: "Start collecting samples for a specific distance",
     parameters: [id: [in: :path, type: :integer, required: true]],
@@ -276,6 +286,31 @@ defmodule Localiser.Web.Controllers.SensorController do
     json(conn, %{status: "calibration_mode", samples_needed: calibration_samples_needed()})
   end
 
+  def calibration_set_tag(conn, %{"id" => id, "tag_id" => tag_id}) when is_binary(tag_id) do
+    sensor = Sensors.get_sensor!(id)
+
+    case SensorServer.set_calibration_tag(sensor.sensor_id, tag_id) do
+      :ok ->
+        json(conn, %{status: "calibration_mode", samples_needed: calibration_samples_needed()})
+
+      {:error, :stage_active} ->
+        conn
+        |> put_status(:unprocessable_entity)
+        |> json(%{error: "cannot change tag while a stage is active"})
+
+      {:error, :not_in_calibration_mode} ->
+        conn
+        |> put_status(:unprocessable_entity)
+        |> json(%{error: "sensor is not in calibration mode"})
+    end
+  end
+
+  def calibration_set_tag(conn, _params) do
+    conn
+    |> put_status(:bad_request)
+    |> json(%{error: "tag_id is required"})
+  end
+
   def calibration_stage_start(conn, %{"id" => id, "distance" => distance})
       when is_number(distance) and distance > 0 do
     sensor = Sensors.get_sensor!(id)
@@ -293,6 +328,11 @@ defmodule Localiser.Web.Controllers.SensorController do
         conn
         |> put_status(:unprocessable_entity)
         |> json(%{error: "sensor is not in calibration mode"})
+
+      {:error, :no_tag_selected} ->
+        conn
+        |> put_status(:unprocessable_entity)
+        |> json(%{error: "no tag selected — call /calibration/tag first"})
     end
   end