Add database indexes and optimize SNMP polling

- Added partial indexes for alerts and SNMP equipment queries
- Fixed missing agent_assignments association in Equipment schema
- Fixed missing preloads to avoid N+1 queries
- Optimized Rust agent SNMP credential field access

lib/towerops/agents.ex

@@ -244,7 +244,7 @@ defmodule Towerops.Agents do
       from(e in Equipment,
         where: e.snmp_enabled == true,
         preload: [
-          :site,
+          :agent_assignments,
           site: :organization,
           snmp_device: [:sensors, :interfaces]
         ]

lib/towerops/equipment/equipment.ex

@@ -27,6 +27,7 @@ defmodule Towerops.Equipment.Equipment do
     belongs_to :site, Towerops.Sites.Site
 
     has_one :snmp_device, Towerops.Snmp.Device
+    has_many :agent_assignments, Towerops.Agents.AgentAssignment
 
     timestamps(type: :utc_datetime)
   end

priv/repo/migrations/20260115011502_add_alerts_equipment_indexes.exs

@@ -0,0 +1,28 @@
+defmodule Towerops.Repo.Migrations.AddAlertsEquipmentIndexes do
+  use Ecto.Migration
+
+  def change do
+    # Composite index for active alerts queries - filters by type and resolved status
+    # Used by list_organization_active_alerts/1 and get_active_alert/2
+    # Partial index only on unresolved alerts for efficiency
+    create_if_not_exists index(:alerts, [:equipment_id, :alert_type, :resolved_at],
+                           where: "resolved_at IS NULL",
+                           name: :idx_alerts_equipment_type_unresolved
+                         )
+
+    # Composite index for SNMP-enabled equipment polling queries
+    # Used by list_snmp_enabled_equipment/0 and agent polling target queries
+    # Partial index only on SNMP-enabled equipment
+    create_if_not_exists index(:equipment, [:snmp_enabled, :last_snmp_poll_at],
+                           where: "snmp_enabled = true",
+                           name: :idx_equipment_snmp_poll
+                         )
+
+    # Composite index for monitoring-enabled equipment with SNMP
+    # Covers common query pattern: WHERE snmp_enabled AND monitoring_enabled
+    create_if_not_exists index(:equipment, [:snmp_enabled, :monitoring_enabled, :site_id],
+                           where: "snmp_enabled = true AND monitoring_enabled = true",
+                           name: :idx_equipment_snmp_monitoring_site
+                         )
+  end
+end