Major refactoring: Updated manifest format and renamed backup to snapshot

- Created manifest.go with proper Manifest structure including blob sizes
- Updated manifest generation to include compressed size for each blob
- Added TotalCompressedSize field to manifest for quick access
- Renamed backup package to snapshot for clarity
- Updated snapshot list to show all remote snapshots
- Remote snapshots not in local DB fetch manifest to get size
- Local snapshots not in remote are automatically deleted
- Removed backwards compatibility code (pre-1.0, no users)
- Fixed prune command to use new manifest format
- Updated all imports and references from backup to snapshot

internal/snapshot/manifest.go

@@ -0,0 +1,70 @@
+package snapshot
+
+import (
+	"bytes"
+	"encoding/json"
+	"fmt"
+	"io"
+
+	"github.com/klauspost/compress/zstd"
+)
+
+// Manifest represents the structure of a snapshot's blob manifest
+type Manifest struct {
+	SnapshotID          string     `json:"snapshot_id"`
+	Timestamp           string     `json:"timestamp"`
+	BlobCount           int        `json:"blob_count"`
+	TotalCompressedSize int64      `json:"total_compressed_size"`
+	Blobs               []BlobInfo `json:"blobs"`
+}
+
+// BlobInfo represents information about a single blob in the manifest
+type BlobInfo struct {
+	Hash           string `json:"hash"`
+	CompressedSize int64  `json:"compressed_size"`
+}
+
+// DecodeManifest decodes a manifest from a reader containing compressed JSON
+func DecodeManifest(r io.Reader) (*Manifest, error) {
+	// Decompress using zstd
+	zr, err := zstd.NewReader(r)
+	if err != nil {
+		return nil, fmt.Errorf("creating zstd reader: %w", err)
+	}
+	defer zr.Close()
+
+	// Decode JSON manifest
+	var manifest Manifest
+	if err := json.NewDecoder(zr).Decode(&manifest); err != nil {
+		return nil, fmt.Errorf("decoding manifest: %w", err)
+	}
+
+	return &manifest, nil
+}
+
+// EncodeManifest encodes a manifest to compressed JSON
+func EncodeManifest(manifest *Manifest, compressionLevel int) ([]byte, error) {
+	// Marshal to JSON
+	jsonData, err := json.MarshalIndent(manifest, "", "  ")
+	if err != nil {
+		return nil, fmt.Errorf("marshaling manifest: %w", err)
+	}
+
+	// Compress using zstd
+	var compressedBuf bytes.Buffer
+	writer, err := zstd.NewWriter(&compressedBuf, zstd.WithEncoderLevel(zstd.EncoderLevelFromZstd(compressionLevel)))
+	if err != nil {
+		return nil, fmt.Errorf("creating zstd writer: %w", err)
+	}
+
+	if _, err := writer.Write(jsonData); err != nil {
+		_ = writer.Close()
+		return nil, fmt.Errorf("writing compressed data: %w", err)
+	}
+
+	if err := writer.Close(); err != nil {
+		return nil, fmt.Errorf("closing zstd writer: %w", err)
+	}
+
+	return compressedBuf.Bytes(), nil
+}