Implement local SQLite index database with repositories

- Add SQLite database connection management with proper error handling
- Implement schema for files, chunks, blobs, and snapshots tables
- Create repository pattern for each database table
- Add transaction support with proper rollback handling
- Integrate database module with fx dependency injection
- Make index path configurable via VAULTIK_INDEX_PATH env var
- Add fatal error handling for database integrity issues
- Update DESIGN.md to clarify file_chunks vs chunk_files distinction
- Remove FinalHash from BlobInfo (blobs are content-addressable)
- Add file metadata support (mtime, ctime, mode, uid, gid, symlinks)

internal/models/models.go

@@ -27,8 +27,7 @@ type ChunkRef struct {
 
 // BlobInfo represents an encrypted blob containing multiple chunks
 type BlobInfo struct {
-	Hash       string    // Hash of encrypted blob
-	FinalHash  string    // Hash after compression and encryption
+	Hash       string // SHA256 hash of the blob content (content-addressable)
 	CreatedAt  time.Time
 	Size       int64
 	ChunkCount int
@@ -36,7 +35,7 @@ type BlobInfo struct {
 
 // Snapshot represents a backup snapshot
 type Snapshot struct {
-	ID           string    // ISO8601 timestamp
+	ID           string // ISO8601 timestamp
 	Hostname     string
 	Version      string
 	CreatedAt    time.Time
@@ -70,4 +69,4 @@ type DirtyPath struct {
 	Path      string
 	MarkedAt  time.Time
 	EventType string // "create", "modify", "delete"
-}
+}