Add batch loading for sensor/interface stats to eliminate N+1 queries

Added two new functions to Snmp context:
- get_latest_sensor_readings_batch/1 - Loads latest readings for multiple sensors in one query
- get_latest_interface_stats_batch/1 - Loads latest stats for multiple interfaces in one query

Both use PostgreSQL DISTINCT ON to efficiently get only the latest record per ID.

Updated EquipmentLive.Show.load_snmp_data/1 to use batch functions:
- Equipment with 20 sensors: 20 queries → 1 query (20x reduction)
- Equipment with 10 interfaces: 10 queries → 1 query (10x reduction)
- Total improvement: 30 queries → 2 queries for typical equipment page load

This significantly improves page load performance for equipment with many sensors/interfaces.

lib/towerops/snmp.ex

@@ -227,6 +227,35 @@ defmodule Towerops.Snmp do
     |> Repo.one()
   end
 
+  @doc """
+  Gets the latest sensor readings for multiple sensors in a single query.
+
+  Returns a map of %{sensor_id => reading} for efficient batch loading.
+  Sensors without readings will not be present in the map.
+
+  ## Examples
+
+      iex> get_latest_sensor_readings_batch([sensor_id1, sensor_id2])
+      %{sensor_id1 => %SensorReading{}, sensor_id2 => %SensorReading{}}
+  """
+  def get_latest_sensor_readings_batch(sensor_ids) when is_list(sensor_ids) do
+    if Enum.empty?(sensor_ids) do
+      %{}
+    else
+      # Use DISTINCT ON to get only the latest reading per sensor
+      query =
+        from(r in SensorReading,
+          where: r.sensor_id in ^sensor_ids,
+          distinct: r.sensor_id,
+          order_by: [asc: r.sensor_id, desc: r.checked_at]
+        )
+
+      query
+      |> Repo.all()
+      |> Map.new(fn reading -> {reading.sensor_id, reading} end)
+    end
+  end
+
   # Interface stats queries
 
   @doc """
@@ -268,6 +297,35 @@ defmodule Towerops.Snmp do
     |> Repo.one()
   end
 
+  @doc """
+  Gets the latest interface stats for multiple interfaces in a single query.
+
+  Returns a map of %{interface_id => stat} for efficient batch loading.
+  Interfaces without stats will not be present in the map.
+
+  ## Examples
+
+      iex> get_latest_interface_stats_batch([if_id1, if_id2])
+      %{if_id1 => %InterfaceStat{}, if_id2 => %InterfaceStat{}}
+  """
+  def get_latest_interface_stats_batch(interface_ids) when is_list(interface_ids) do
+    if Enum.empty?(interface_ids) do
+      %{}
+    else
+      # Use DISTINCT ON to get only the latest stat per interface
+      query =
+        from(s in InterfaceStat,
+          where: s.interface_id in ^interface_ids,
+          distinct: s.interface_id,
+          order_by: [asc: s.interface_id, desc: s.checked_at]
+        )
+
+      query
+      |> Repo.all()
+      |> Map.new(fn stat -> {stat.interface_id, stat} end)
+    end
+  end
+
   @doc """
   Records a new sensor reading.
   """

lib/towerops_web/live/equipment_live/show.ex

@@ -215,17 +215,23 @@ defmodule ToweropsWeb.EquipmentLive.Show do
         %{device: nil, interfaces: [], sensors: []}
 
       device ->
-        # Preload latest readings for sensors
+        # Batch load latest readings for all sensors in a single query
+        sensor_ids = Enum.map(device.sensors, & &1.id)
+        latest_readings_map = Snmp.get_latest_sensor_readings_batch(sensor_ids)
+
         sensors_with_readings =
           Enum.map(device.sensors, fn sensor ->
-            latest_reading = Snmp.get_latest_sensor_reading(sensor.id)
+            latest_reading = Map.get(latest_readings_map, sensor.id)
             Map.put(sensor, :latest_reading, latest_reading)
           end)
 
-        # Preload latest stats for interfaces
+        # Batch load latest stats for all interfaces in a single query
+        interface_ids = Enum.map(device.interfaces, & &1.id)
+        latest_stats_map = Snmp.get_latest_interface_stats_batch(interface_ids)
+
         interfaces_with_stats =
           Enum.map(device.interfaces, fn interface ->
-            latest_stat = Snmp.get_latest_interface_stat(interface.id)
+            latest_stat = Map.get(latest_stats_map, interface.id)
             Map.put(interface, :latest_stat, latest_stat)
           end)