fix: handle duplicate InvitationLogEntry on retry

Second attempt at fixing the "Member has already been taken" validation error
when re-sending workshop invitations.

Problem:
The previous fix (PR #2556) used find_or_initialize_by with the status included
in the query. However, the InvitationLogEntry uniqueness constraint only validates
(member_id, invitation_type, invitation_id), NOT status. This means:

- A member can have ONE success entry AND ONE failure entry for the same invitation
- find_or_initialize_by(member, invitation, :success) would return a persisted
  entry with status=:failed if one existed, causing the wrong status to be used
- Or if building a new entry with status=:success when status=:failed already
  existed, the uniqueness validation would fail

Fix:
1. Split the check: first find any existing entry by (member, invitation),
   ignoring status
2. If found, return it (either success or failure)
3. Only if no entry exists, create a new one with the specified status

Additionally, add autosave: false to the has_many :entries association to prevent
Rails from attempting to autosave unpersisted entries when fail_batch is called,
which was causing "Validation failed: Entries is invalid" errors.

Changes:
- app/services/invitation_logger.rb: Separate find_or_build_entry logic
- app/models/invitation_log.rb: Add autosave: false to entries association

Tests:
- All 41 existing tests pass
- Retry behavior tests verify no duplicate entries are created

Author: mroderick

app/models/invitation_log.rb

@@ -6,7 +6,7 @@ class InvitationLog < ApplicationRecord
   belongs_to :loggable, polymorphic: true
   belongs_to :initiator, class_name: 'Member', optional: true
   belongs_to :chapter, optional: true
-  has_many :entries, class_name: 'InvitationLogEntry', dependent: :destroy
+  has_many :entries, class_name: 'InvitationLogEntry', dependent: :destroy, autosave: false
 
   before_create :set_expires_at
 

app/services/invitation_logger.rb

@@ -78,10 +78,13 @@ def fail_batch(error)
   private
 
   def find_or_build_entry(member, invitation, status)
-    @log.entries.find_or_initialize_by(
-      member:,
-      invitation:,
-      status:
+    existing_entry = @log.entries.find_by(member: member, invitation: invitation)
+    return existing_entry if existing_entry
+
+    @log.entries.new(
+      member: member,
+      invitation: invitation,
+      status: status
     )
   end