fix(polls): fetch existing CloudKit record before updating vote

When updating a vote, CloudKit requires the server's changeTag to modify
existing records. Creating a new CKRecord caused "record to insert already
exists" errors. Now fetches the existing record first before saving.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

SportsTime/Core/Services/PollService.swift

@@ -234,13 +234,26 @@ actor PollService {
             throw PollError.notPollOwner  // Using this error for now - voter can only update own vote
         }
 
-        var updatedVote = vote
+        // Fetch the existing record to get the server's changeTag
-        updatedVote.modifiedAt = Date()
+        let recordID = CKRecord.ID(recordName: vote.id.uuidString)
+        let existingRecord: CKRecord
+        do {
+            existingRecord = try await publicDatabase.record(for: recordID)
+        } catch let error as CKError {
+            throw mapCloudKitError(error)
+        } catch {
+            throw PollError.unknown(error)
+        }
 
-        let ckVote = CKPollVote(vote: updatedVote)
+        // Update the fields on the fetched record
+        let now = Date()
+        existingRecord[CKPollVote.rankingsKey] = vote.rankings
+        existingRecord[CKPollVote.modifiedAtKey] = now
 
         do {
-            try await publicDatabase.save(ckVote.record)
+            try await publicDatabase.save(existingRecord)
+            var updatedVote = vote
+            updatedVote.modifiedAt = now
             return updatedVote
         } catch let error as CKError {
             throw mapCloudKitError(error)