Member/CycleGenerator: better delete_cycles errors; UUID-based advisory lock

delete_cycles returns first error for debugging. Advisory lock key
derived from member id (first 8 bytes of UUID hex) to reduce
phash2 collision risk; fallback to phash2 on invalid UUID.

lib/mv/membership_fees/cycle_generator.ex

@@ -112,7 +112,7 @@ defmodule Mv.MembershipFees.CycleGenerator do
   end
 
   defp do_generate_cycles_with_lock(member, today, false, opts) do
-    lock_key = :erlang.phash2(member.id)
+    lock_key = Member.advisory_lock_key_for_member_id(member.id)
 
     Repo.transaction(fn ->
       EctoSQL.query!(Repo, "SELECT pg_advisory_xact_lock($1)", [lock_key])