feat: Step 5 - Implement CreateIncrementalBackup() for PostgreSQL

Implemented full incremental backup creation:

internal/backup/incremental_postgres.go:
- CreateIncrementalBackup() - main entry point
- Validates base backup exists and is full backup
- Loads base backup metadata (.meta.json)
- Uses FindChangedFiles() to detect modifications
- Creates tar.gz with ONLY changed files
- Generates incremental metadata with:
  - Base backup ID (SHA-256)
  - Backup chain (base -> incr1 -> incr2...)
  - Changed file count and total size
- Saves .meta.json with full incremental metadata
- Calculates SHA-256 checksum of archive

internal/backup/incremental_tar.go:
- createTarGz() - creates compressed archive
- addFileToTar() - adds individual files to tar
- Handles context cancellation
- Progress logging for each file
- Preserves file permissions and timestamps

Helper Functions:
- loadBackupInfo() - loads BackupMetadata from .meta.json
- buildBackupChain() - constructs restore chain
- CalculateFileChecksum() - SHA-256 for archive

Features:
✅ Creates tar.gz with ONLY changed files
✅ Much smaller than full backup
✅ Links to base backup via SHA-256
✅ Tracks complete restore chain
✅ Full metadata for restore validation
✅ Context-aware (cancellable)

Status: READY FOR TESTING
Next: Wire into backup engine, test with real PostgreSQL data

internal/backup/incremental_tar.go

@@ -0,0 +1,95 @@
+package backup
+
+import (
+	"archive/tar"
+	"compress/gzip"
+	"context"
+	"fmt"
+	"io"
+	"os"
+)
+
+// createTarGz creates a tar.gz archive with the specified changed files
+func (e *PostgresIncrementalEngine) createTarGz(ctx context.Context, outputFile string, changedFiles []ChangedFile, config *IncrementalBackupConfig) error {
+	// Create output file
+	outFile, err := os.Create(outputFile)
+	if err != nil {
+		return fmt.Errorf("failed to create output file: %w", err)
+	}
+	defer outFile.Close()
+
+	// Create gzip writer
+	gzWriter, err := gzip.NewWriterLevel(outFile, config.CompressionLevel)
+	if err != nil {
+		return fmt.Errorf("failed to create gzip writer: %w", err)
+	}
+	defer gzWriter.Close()
+
+	// Create tar writer
+	tarWriter := tar.NewWriter(gzWriter)
+	defer tarWriter.Close()
+
+	// Add each changed file to archive
+	for i, changedFile := range changedFiles {
+		// Check context cancellation
+		select {
+		case <-ctx.Done():
+			return ctx.Err()
+		default:
+		}
+
+		e.log.Debug("Adding file to archive",
+			"file", changedFile.RelativePath,
+			"progress", fmt.Sprintf("%d/%d", i+1, len(changedFiles)))
+
+		if err := e.addFileToTar(tarWriter, changedFile); err != nil {
+			return fmt.Errorf("failed to add file %s: %w", changedFile.RelativePath, err)
+		}
+	}
+
+	return nil
+}
+
+// addFileToTar adds a single file to the tar archive
+func (e *PostgresIncrementalEngine) addFileToTar(tarWriter *tar.Writer, changedFile ChangedFile) error {
+	// Open the file
+	file, err := os.Open(changedFile.AbsolutePath)
+	if err != nil {
+		return fmt.Errorf("failed to open file: %w", err)
+	}
+	defer file.Close()
+
+	// Get file info
+	info, err := file.Stat()
+	if err != nil {
+		return fmt.Errorf("failed to stat file: %w", err)
+	}
+
+	// Skip if file has been deleted/changed since scan
+	if info.Size() != changedFile.Size {
+		e.log.Warn("File size changed since scan, using current size",
+			"file", changedFile.RelativePath,
+			"old_size", changedFile.Size,
+			"new_size", info.Size())
+	}
+
+	// Create tar header
+	header := &tar.Header{
+		Name:    changedFile.RelativePath,
+		Size:    info.Size(),
+		Mode:    int64(info.Mode()),
+		ModTime: info.ModTime(),
+	}
+
+	// Write header
+	if err := tarWriter.WriteHeader(header); err != nil {
+		return fmt.Errorf("failed to write tar header: %w", err)
+	}
+
+	// Copy file content
+	if _, err := io.Copy(tarWriter, file); err != nil {
+		return fmt.Errorf("failed to copy file content: %w", err)
+	}
+
+	return nil
+}