debug: add comprehensive logging for merge groups functionality

Add debug logs to diagnose merge not working issue:

1. batchMergeGroups():
   - Log selected groupIds
   - Log mergeCandidateGroups count

2. executeMerge():
   - Log targetGroupId and targetGroup
   - Log targetTraceId type and value
   - Log targetTrace data
   - Log sourceGroups count
   - Log totalTraces and validTraces
   - Log each merge_trace API call params
   - Log success/fail counts

Purpose: Identify why merge fails when move works
Next: Check browser console for specific errors

src/views/PeopleView.vue

@@ -1706,23 +1706,31 @@ async function batchMergeGroups() {
   }
   
   const groupIds = Array.from(selectedGroups.value)
+  console.log('[batchMergeGroups] groupIds:', groupIds)
+  
   mergeCandidateGroups.value = clusterAsPending.value.filter((c: any) => groupIds.includes(c.identity_uuid))
+  console.log('[batchMergeGroups] mergeCandidateGroups:', mergeCandidateGroups.value.length)
+  
   showMergeTargetModal.value = true
 }
 
 async function executeMerge(targetGroupId: string) {
+  console.log('[executeMerge] targetGroupId:', targetGroupId)
   showMergeTargetModal.value = false
   
   const targetGroup = clusterAsPending.value.find((c: any) => c.identity_uuid === targetGroupId)
+  console.log('[executeMerge] targetGroup:', targetGroup)
   if (!targetGroup) return
   
   const targetTraceId = targetGroup.trace_ids?.[0]
+  console.log('[executeMerge] targetTraceId:', targetTraceId, typeof targetTraceId)
   if (!targetTraceId) {
     alert('Target group has no traces')
     return
   }
   
   const targetTrace = clusterTracesMap.value[String(targetTraceId)]
+  console.log('[executeMerge] targetTrace:', targetTrace)
   const targetFileUuid = targetTrace?.file_uuid
   if (!targetFileUuid) {
     alert('Target trace has no file_uuid')
@@ -1732,6 +1740,7 @@ async function executeMerge(targetGroupId: string) {
   const groupIds = Array.from(selectedGroups.value)
   const sourceGroupIds = groupIds.filter(id => id !== targetGroupId)
   const sourceGroups = clusterAsPending.value.filter((c: any) => sourceGroupIds.includes(c.identity_uuid))
+  console.log('[executeMerge] sourceGroups:', sourceGroups.length)
   
   let totalTraces = 0
   let validTraces = 0
@@ -1743,6 +1752,8 @@ async function executeMerge(targetGroupId: string) {
     }
   }
   
+  console.log('[executeMerge] totalTraces:', totalTraces, 'validTraces:', validTraces)
+  
   if (validTraces === 0) {
     alert('No valid traces to merge (all missing file_uuid)')
     return
@@ -1753,31 +1764,45 @@ async function executeMerge(targetGroupId: string) {
   operationLoading.value = true
   operationMessage.value = `Merging ${validTraces} trace(s)...`
   
+  let successCount = 0
+  let failCount = 0
+  
   for (const g of sourceGroups) {
     for (const srcTraceId of g.trace_ids || []) {
       const srcTrace = clusterTracesMap.value[String(srcTraceId)]
       const srcFileUuid = srcTrace?.file_uuid
       if (!srcFileUuid) {
         console.warn('[merge] skipping trace without file_uuid:', srcTraceId)
+        failCount++
         continue
       }
       if (srcFileUuid !== targetFileUuid) {
         console.warn('[merge] skipping cross-file trace:', srcTraceId, srcFileUuid, '!=', targetFileUuid)
+        failCount++
         continue
       }
       try {
+        console.log('[executeMerge] calling merge_trace:', {
+          file_uuid: srcFileUuid,
+          source_id: srcTraceId,
+          target_id: targetTraceId
+        })
         await apiCall('merge_trace', { 
           file_uuid: srcFileUuid, 
           source_id: srcTraceId, 
           target_id: targetTraceId 
         })
+        successCount++
         updateClusterAfterMerge(g.identity_uuid, targetGroupId)
       } catch (e) {
         console.error('Merge trace failed:', srcTraceId, e)
+        failCount++
       }
     }
   }
   
+  console.log('[executeMerge] completed:', successCount, 'success,', failCount, 'failed')
+  
   operationLoading.value = false
   operationMessage.value = ''