fix: implement missing sync service methods for data replication

- Implemented fetchSingleKVFromPeer: HTTP client to fetch KV pairs from peers
- Implemented getLocalData: Badger DB access for local data retrieval
- Implemented deleteKVLocally: Local deletion with timestamp index cleanup
- Implemented storeReplicatedDataWithMetadata: Preserves original UUID/timestamp
- Implemented resolveConflict: Simple conflict resolution (newer timestamp wins)
- Implemented fetchAndStoreRange: Fetches KV ranges for Merkle sync

This fixes the critical data replication issue where sync was failing with
"not implemented" errors. Integration tests now pass for data replication.

🤖 Generated with [Claude Code](https://claude.ai/code)

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

cluster/sync.go

@@ -292,30 +292,108 @@ func (s *SyncService) handleLeafLevelDiff(peerAddress string, keys []string, loc
 	}
 }
 
-// Add placeholder methods that would need to be implemented or injected
-func (s *SyncService) fetchSingleKVFromPeer(peerAddress, key string) (*types.StoredValue, error) {
-	// This would be implemented similar to the main.go version
-	return nil, fmt.Errorf("not implemented")
+// fetchSingleKVFromPeer fetches a single KV pair from a peer
+func (s *SyncService) fetchSingleKVFromPeer(peerAddress, path string) (*types.StoredValue, error) {
+	client := &http.Client{Timeout: 5 * time.Second}
+	url := fmt.Sprintf("http://%s/kv/%s", peerAddress, path)
+
+	resp, err := client.Get(url)
+	if err != nil {
+		return nil, err
+	}
+	defer resp.Body.Close()
+
+	if resp.StatusCode == http.StatusNotFound {
+		return nil, nil // Key might have been deleted on the peer
+	}
+	if resp.StatusCode != http.StatusOK {
+		return nil, fmt.Errorf("peer returned status %d for path %s", resp.StatusCode, path)
+	}
+
+	var storedValue types.StoredValue
+	if err := json.NewDecoder(resp.Body).Decode(&storedValue); err != nil {
+		return nil, fmt.Errorf("failed to decode types.StoredValue from peer: %v", err)
+	}
+
+	return &storedValue, nil
 }
 
-func (s *SyncService) getLocalData(key string) (*types.StoredValue, bool) {
-	// This would be implemented similar to the main.go version
-	return nil, false
+// getLocalData is a utility to retrieve a types.StoredValue from local DB
+func (s *SyncService) getLocalData(path string) (*types.StoredValue, bool) {
+	var storedValue types.StoredValue
+	err := s.db.View(func(txn *badger.Txn) error {
+		item, err := txn.Get([]byte(path))
+		if err != nil {
+			return err
+		}
+
+		return item.Value(func(val []byte) error {
+			return json.Unmarshal(val, &storedValue)
+		})
+	})
+
+	if err != nil {
+		return nil, false
+	}
+
+	return &storedValue, true
 }
 
+// deleteKVLocally deletes a key-value pair and its associated timestamp index locally
 func (s *SyncService) deleteKVLocally(key string, timestamp int64) error {
-	// This would be implemented similar to the main.go version
-	return fmt.Errorf("not implemented")
+	return s.db.Update(func(txn *badger.Txn) error {
+		// Delete the main key
+		if err := txn.Delete([]byte(key)); err != nil {
+			return err
+		}
+
+		// Delete the timestamp index
+		indexKey := fmt.Sprintf("_ts:%d:%s", timestamp, key)
+		return txn.Delete([]byte(indexKey))
+	})
 }
 
-func (s *SyncService) storeReplicatedDataWithMetadata(key string, value *types.StoredValue) error {
-	// This would be implemented similar to the main.go version
-	return fmt.Errorf("not implemented")
+// storeReplicatedDataWithMetadata stores replicated data preserving its original metadata
+func (s *SyncService) storeReplicatedDataWithMetadata(path string, storedValue *types.StoredValue) error {
+	valueBytes, err := json.Marshal(storedValue)
+	if err != nil {
+		return err
+	}
+
+	return s.db.Update(func(txn *badger.Txn) error {
+		// Store main data
+		if err := txn.Set([]byte(path), valueBytes); err != nil {
+			return err
+		}
+
+		// Store timestamp index
+		indexKey := fmt.Sprintf("_ts:%020d:%s", storedValue.Timestamp, path)
+		return txn.Set([]byte(indexKey), []byte(storedValue.UUID))
+	})
 }
 
+// resolveConflict performs simple conflict resolution (newer timestamp wins)
 func (s *SyncService) resolveConflict(key string, local, remote *types.StoredValue, peerAddress string) error {
-	// This would be implemented similar to the main.go version
-	return fmt.Errorf("not implemented")
+	s.logger.WithFields(logrus.Fields{
+		"key":         key,
+		"local_ts":    local.Timestamp,
+		"remote_ts":   remote.Timestamp,
+		"local_uuid":  local.UUID,
+		"remote_uuid": remote.UUID,
+	}).Info("Resolving timestamp collision conflict")
+
+	if remote.Timestamp > local.Timestamp {
+		// Remote is newer, store it
+		err := s.storeReplicatedDataWithMetadata(key, remote)
+		if err == nil {
+			s.logger.WithField("key", key).Info("Conflict resolved: remote data wins (newer)")
+		}
+		return err
+	}
+
+	// Local is newer or equal, keep local data
+	s.logger.WithField("key", key).Info("Conflict resolved: local data wins (newer or equal)")
+	return nil
 }
 
 // requestMerkleDiff requests children hashes or keys for a given node/range from a peer
@@ -383,6 +461,47 @@ func (s *SyncService) handleChildrenDiff(peerAddress string, children []types.Me
 
 // fetchAndStoreRange fetches a range of KV pairs from a peer and stores them locally
 func (s *SyncService) fetchAndStoreRange(peerAddress string, startKey, endKey string) error {
-	// This would be implemented similar to the main.go version
-	return fmt.Errorf("not implemented")
+	req := types.KVRangeRequest{
+		StartKey: startKey,
+		EndKey:   endKey,
+		Limit:    0, // No limit
+	}
+	jsonData, err := json.Marshal(req)
+	if err != nil {
+		return err
+	}
+
+	client := &http.Client{Timeout: 30 * time.Second} // Longer timeout for range fetches
+	url := fmt.Sprintf("http://%s/kv_range", peerAddress)
+
+	resp, err := client.Post(url, "application/json", bytes.NewBuffer(jsonData))
+	if err != nil {
+		return err
+	}
+	defer resp.Body.Close()
+
+	if resp.StatusCode != http.StatusOK {
+		return fmt.Errorf("peer returned status %d for KV range fetch", resp.StatusCode)
+	}
+
+	var rangeResp types.KVRangeResponse
+	if err := json.NewDecoder(resp.Body).Decode(&rangeResp); err != nil {
+		return err
+	}
+
+	for _, pair := range rangeResp.Pairs {
+		// Use storeReplicatedDataWithMetadata to preserve original UUID/Timestamp
+		if err := s.storeReplicatedDataWithMetadata(pair.Path, &pair.StoredValue); err != nil {
+			s.logger.WithError(err).WithFields(logrus.Fields{
+				"peer": peerAddress,
+				"path": pair.Path,
+			}).Error("Failed to store fetched range data")
+		} else {
+			s.logger.WithFields(logrus.Fields{
+				"peer": peerAddress,
+				"path": pair.Path,
+			}).Debug("Stored data from fetched range")
+		}
+	}
+	return nil
 }