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/database/models.go

@@ -0,0 +1,67 @@
+package database
+
+import "time"
+
+// File represents a file record in the database
+type File struct {
+	Path       string
+	MTime      time.Time
+	CTime      time.Time
+	Size       int64
+	Mode       uint32
+	UID        uint32
+	GID        uint32
+	LinkTarget string // empty for regular files, target path for symlinks
+}
+
+// IsSymlink returns true if this file is a symbolic link
+func (f *File) IsSymlink() bool {
+	return f.LinkTarget != ""
+}
+
+// FileChunk represents the mapping between files and chunks
+type FileChunk struct {
+	Path      string
+	Idx       int
+	ChunkHash string
+}
+
+// Chunk represents a chunk record in the database
+type Chunk struct {
+	ChunkHash string
+	SHA256    string
+	Size      int64
+}
+
+// Blob represents a blob record in the database
+type Blob struct {
+	BlobHash  string
+	CreatedTS time.Time
+}
+
+// BlobChunk represents the mapping between blobs and chunks
+type BlobChunk struct {
+	BlobHash  string
+	ChunkHash string
+	Offset    int64
+	Length    int64
+}
+
+// ChunkFile represents the reverse mapping of chunks to files
+type ChunkFile struct {
+	ChunkHash  string
+	FilePath   string
+	FileOffset int64
+	Length     int64
+}
+
+// Snapshot represents a snapshot record in the database
+type Snapshot struct {
+	ID             string
+	Hostname       string
+	VaultikVersion string
+	CreatedTS      time.Time
+	FileCount      int64
+	ChunkCount     int64
+	BlobCount      int64
+}