refactor: use sql_sandbox config instead of env for sync/async

- Replace Application.get_env(:mv, :env) with :sql_sandbox config
- Remove redundant :env config from test.exs
- More explicit and less error-prone for test environment detection

config/test.exs

@@ -47,7 +47,5 @@ config :mv, :session_identifier, :unsafe
 config :mv, :require_token_presence_for_authentication, false
 
 # Enable SQL Sandbox for async LiveView tests
+# This flag controls sync vs async behavior in CycleGenerator after_action hooks
 config :mv, :sql_sandbox, true
-
-# Mark test environment for conditional behavior (e.g., sync vs async operations)
-config :mv, :env, :test

lib/membership/member.ex

@@ -112,7 +112,7 @@ defmodule Mv.Membership.Member do
       # but in test environment it runs synchronously for DB sandbox compatibility
       change after_action(fn _changeset, member, _context ->
                if member.membership_fee_type_id && member.join_date do
-                 if Application.get_env(:mv, :env) == :test do
+                 if Application.get_env(:mv, :sql_sandbox, false) do
                    # Run synchronously in test environment for DB sandbox compatibility
                    Mv.MembershipFees.CycleGenerator.generate_cycles_for_member(member.id)
                  else
@@ -180,7 +180,7 @@ defmodule Mv.Membership.Member do
                  Ash.Changeset.changing_attribute?(changeset, :membership_fee_type_id)
 
                if fee_type_changed && member.membership_fee_type_id && member.join_date do
-                 if Application.get_env(:mv, :env) == :test do
+                 if Application.get_env(:mv, :sql_sandbox, false) do
                    # Run synchronously in test environment for DB sandbox compatibility
                    Mv.MembershipFees.CycleGenerator.generate_cycles_for_member(member.id)
                  else