Optimize list_agent_polling_targets to use SQL filtering

Rewrote list_agent_polling_targets/1 to filter equipment using SQL WHERE clause
instead of loading all SNMP-enabled equipment into memory and filtering in Elixir.

Previous implementation:
- Loaded ALL SNMP-enabled equipment with preloads (potentially 10,000+ records)
- Filtered in application code with equipment_matches_agent?/2 helper
- Called get_equipment_assignment/1 for each piece of equipment (N+1 queries)
- Memory-intensive and slow at scale

New implementation:
- Single SQL query with LEFT JOIN and compound WHERE clause
- Evaluates hierarchical assignment (equipment → site → organization) in SQL
- Only loads equipment assigned to the specific agent
- Scales efficiently to 10,000+ equipment

Performance improvement:
- With 10,000 equipment, 1000 assigned to agent:
  - Before: Load 10,000 records + 10,000 queries = massive memory + slow
  - After: Load 1,000 records in 1 query = 10x memory reduction + instant

Removed unused helper functions:
- equipment_matches_agent?/2
- site_matches_agent?/2

All 40 agent tests passing.

lib/towerops/agents.ex

@@ -239,55 +239,27 @@ defmodule Towerops.Agents do
 
   """
   def list_agent_polling_targets(agent_token_id) do
-    # Get all equipment with necessary preloads
-    equipment_query =
+    # Use SQL to filter equipment by effective agent assignment
+    # This avoids loading all equipment into memory and filtering in application code
+    Repo.all(
       from(e in Equipment,
-        where: e.snmp_enabled == true,
+        join: s in assoc(e, :site),
+        join: o in assoc(s, :organization),
+        left_join: aa in AgentAssignment,
+        on: aa.equipment_id == e.id and aa.enabled == true,
+        where:
+          e.snmp_enabled == true and
+            (aa.agent_token_id == ^agent_token_id or
+               (is_nil(aa.agent_token_id) and s.agent_token_id == ^agent_token_id) or
+               (is_nil(aa.agent_token_id) and is_nil(s.agent_token_id) and
+                  o.default_agent_token_id == ^agent_token_id)),
         preload: [
           :agent_assignments,
           site: :organization,
           snmp_device: [:sensors, :interfaces]
         ]
       )
-
-    equipment_list = Repo.all(equipment_query)
-
-    # Filter to only equipment that resolves to this agent
-    Enum.filter(equipment_list, fn equipment ->
-      equipment_matches_agent?(equipment, agent_token_id)
-    end)
-  end
-
-  defp equipment_matches_agent?(equipment, agent_token_id) do
-    # Check direct assignment first
-    case get_equipment_assignment(equipment.id) do
-      %AgentAssignment{agent_token_id: ^agent_token_id, enabled: true} ->
-        true
-
-      %AgentAssignment{} ->
-        # Has a different direct assignment
-        false
-
-      nil ->
-        # No direct assignment, check site then org
-        site = equipment.site
-        site_matches_agent?(site, agent_token_id)
-    end
-  end
-
-  defp site_matches_agent?(site, agent_token_id) do
-    cond do
-      # Check site agent
-      site.agent_token_id == agent_token_id ->
-        true
-
-      # Check org default (only if site doesn't have an agent set)
-      is_nil(site.agent_token_id) and site.organization.default_agent_token_id == agent_token_id ->
-        true
-
-      true ->
-        false
-    end
+    )
   end
 
   @doc """