Add deterministic deduplication, rclone backend, and database purge command

- Implement deterministic blob hashing using double SHA256 of uncompressed
  plaintext data, enabling deduplication even after local DB is cleared
- Add Stat() check before blob upload to skip existing blobs in storage
- Add rclone storage backend for additional remote storage options
- Add 'vaultik database purge' command to erase local state DB
- Add 'vaultik remote check' command to verify remote connectivity
- Show configured snapshots in 'vaultik snapshot list' output
- Skip macOS resource fork files (._*) when listing remote snapshots
- Use multi-threaded zstd compression (CPUs - 2 threads)
- Add writer tests for double hashing behavior

internal/cli/database.go

@@ -0,0 +1,102 @@
+package cli
+
+import (
+	"fmt"
+	"os"
+
+	"git.eeqj.de/sneak/vaultik/internal/config"
+	"git.eeqj.de/sneak/vaultik/internal/log"
+	"github.com/spf13/cobra"
+)
+
+// NewDatabaseCommand creates the database command group
+func NewDatabaseCommand() *cobra.Command {
+	cmd := &cobra.Command{
+		Use:   "database",
+		Short: "Manage the local state database",
+		Long:  `Commands for managing the local SQLite state database.`,
+	}
+
+	cmd.AddCommand(
+		newDatabasePurgeCommand(),
+	)
+
+	return cmd
+}
+
+// newDatabasePurgeCommand creates the database purge command
+func newDatabasePurgeCommand() *cobra.Command {
+	var force bool
+
+	cmd := &cobra.Command{
+		Use:   "purge",
+		Short: "Delete the local state database",
+		Long: `Completely removes the local SQLite state database.
+
+This will erase all local tracking of:
+- File metadata and change detection state
+- Chunk and blob mappings
+- Local snapshot records
+
+The remote storage is NOT affected. After purging, the next backup will
+perform a full scan and re-deduplicate against existing remote blobs.
+
+Use --force to skip the confirmation prompt.`,
+		Args: cobra.NoArgs,
+		RunE: func(cmd *cobra.Command, args []string) error {
+			// Resolve config path
+			configPath, err := ResolveConfigPath()
+			if err != nil {
+				return err
+			}
+
+			// Load config to get database path
+			cfg, err := config.Load(configPath)
+			if err != nil {
+				return fmt.Errorf("failed to load config: %w", err)
+			}
+
+			dbPath := cfg.IndexPath
+
+			// Check if database exists
+			if _, err := os.Stat(dbPath); os.IsNotExist(err) {
+				fmt.Printf("Database does not exist: %s\n", dbPath)
+				return nil
+			}
+
+			// Confirm unless --force
+			if !force {
+				fmt.Printf("This will delete the local state database at:\n  %s\n\n", dbPath)
+				fmt.Print("Are you sure? Type 'yes' to confirm: ")
+				var confirm string
+				if _, err := fmt.Scanln(&confirm); err != nil || confirm != "yes" {
+					fmt.Println("Aborted.")
+					return nil
+				}
+			}
+
+			// Delete the database file
+			if err := os.Remove(dbPath); err != nil {
+				return fmt.Errorf("failed to delete database: %w", err)
+			}
+
+			// Also delete WAL and SHM files if they exist
+			walPath := dbPath + "-wal"
+			shmPath := dbPath + "-shm"
+			_ = os.Remove(walPath) // Ignore errors - files may not exist
+			_ = os.Remove(shmPath)
+
+			rootFlags := GetRootFlags()
+			if !rootFlags.Quiet {
+				fmt.Printf("Database purged: %s\n", dbPath)
+			}
+
+			log.Info("Local state database purged", "path", dbPath)
+			return nil
+		},
+	}
+
+	cmd.Flags().BoolVar(&force, "force", false, "Skip confirmation prompt")
+
+	return cmd
+}