refactor: improve cycle generation code quality and documentation

- Remove Process.sleep calls from integration tests (tests run synchronously in SQL sandbox)
- Improve error handling: membership_fee_type_not_found now returns changeset error instead of just logging
- Clarify partial_failure documentation: successful_cycles are not persisted on rollback
- Update documentation: joined_at → join_date, left_at → exit_date
- Document PostgreSQL advisory locks per member (not whole table lock)
- Document gap handling: explicitly deleted cycles are not recreated

test/membership_fees/member_cycle_integration_test.exs

@@ -58,9 +58,6 @@ defmodule Mv.MembershipFees.MemberCycleIntegrationTest do
         })
         |> Ash.create!()
 
-      # Wait for async cycle generation
-      Process.sleep(300)
-
       cycles = get_member_cycles(member.id)
 
       # Should have cycles for 2023 and 2024 (and possibly current year)
@@ -89,9 +86,6 @@ defmodule Mv.MembershipFees.MemberCycleIntegrationTest do
         })
         |> Ash.create!()
 
-      # Wait for potential async cycle generation
-      Process.sleep(200)
-
       cycles = get_member_cycles(member.id)
 
       assert cycles == []
@@ -112,9 +106,6 @@ defmodule Mv.MembershipFees.MemberCycleIntegrationTest do
         })
         |> Ash.create!()
 
-      # Wait for potential async cycle generation
-      Process.sleep(200)
-
       cycles = get_member_cycles(member.id)
 
       assert cycles == []
@@ -145,9 +136,6 @@ defmodule Mv.MembershipFees.MemberCycleIntegrationTest do
       |> Ash.Changeset.for_update(:update_member, %{membership_fee_type_id: fee_type.id})
       |> Ash.update!()
 
-      # Wait for async cycle generation
-      Process.sleep(300)
-
       cycles = get_member_cycles(member.id)
 
       # Should have generated cycles
@@ -178,9 +166,6 @@ defmodule Mv.MembershipFees.MemberCycleIntegrationTest do
 
       members = Enum.map(tasks, &Task.await/1)
 
-      # Wait for all async cycle generations
-      Process.sleep(500)
-
       # Each member should have cycles
       Enum.each(members, fn member ->
         cycles = get_member_cycles(member.id)
@@ -205,9 +190,6 @@ defmodule Mv.MembershipFees.MemberCycleIntegrationTest do
         })
         |> Ash.create!()
 
-      # Wait for async cycle generation
-      Process.sleep(300)
-
       initial_cycles = get_member_cycles(member.id)
       initial_count = length(initial_cycles)