feat: improve error handling in CycleGenerator

- Handle Task crashes in async_stream with {:exit, reason}
- Return {:error, {:partial_failure, successes, errors}} when some cycles fail
- Previously returned {:ok, successful} even on partial failures
- Improves debuggability and allows callers to handle partial failures

lib/mv/membership_fees/cycle_generator.ex

@@ -142,7 +142,15 @@ defmodule Mv.MembershipFees.CycleGenerator do
     |> Task.async_stream(fn member ->
       {member.id, generate_cycles_for_member(member, today: today)}
     end)
-    |> Enum.map(fn {:ok, result} -> result end)
+    |> Enum.map(fn
+      {:ok, result} ->
+        result
+
+      {:exit, reason} ->
+        # Task crashed - log and return error tuple
+        Logger.error("Task crashed during cycle generation: #{inspect(reason)}")
+        {nil, {:error, {:task_exit, reason}}}
+    end)
   end
 
   defp build_results_summary(results) do
@@ -298,7 +306,7 @@ defmodule Mv.MembershipFees.CycleGenerator do
   end
 
   defp create_cycles(cycle_starts, member_id, fee_type_id, amount) do
-    cycles =
+    results =
       Enum.map(cycle_starts, fn cycle_start ->
         attrs = %{
           cycle_start: cycle_start,
@@ -314,15 +322,16 @@ defmodule Mv.MembershipFees.CycleGenerator do
         end
       end)
 
-    errors = Enum.filter(cycles, &match?({:error, _}, &1))
+    {successes, errors} = Enum.split_with(results, &match?({:ok, _}, &1))
+    successful_cycles = Enum.map(successes, fn {:ok, cycle} -> cycle end)
 
     if Enum.empty?(errors) do
-      {:ok, Enum.map(cycles, fn {:ok, cycle} -> cycle end)}
+      {:ok, successful_cycles}
     else
       Logger.warning("Some cycles failed to create: #{inspect(errors)}")
-      # Return successfully created cycles anyway
-      successful = Enum.filter(cycles, &match?({:ok, _}, &1)) |> Enum.map(fn {:ok, c} -> c end)
-      {:ok, successful}
+      # Return partial failure with both successful and failed cycles
+      # This allows callers to decide how to handle partial failures
+      {:error, {:partial_failure, successful_cycles, errors}}
     end
   end
 end