Store original documents for download after ingestion

Persist uploaded files to {data_dir}/documents/{content_hash}{ext} after
successful ingestion. Add GET /documents/{id}/file endpoint for retrieval,
delete stored files on document deletion, and add `kb export` client command.
Includes schema migration, tests, and spec updates.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

client/cmd/export.go

@@ -0,0 +1,74 @@
+package cmd
+
+import (
+	"fmt"
+	"io"
+	"mime"
+	"os"
+	"path/filepath"
+
+	"github.com/kb-search/kb/internal/api"
+	"github.com/spf13/cobra"
+)
+
+var exportCmd = &cobra.Command{
+	Use:   "export <id>",
+	Short: "Download original document file",
+	Args:  cobra.ExactArgs(1),
+	RunE:  runExport,
+}
+
+func init() {
+	exportCmd.Flags().StringP("output", "o", "", "output file path (default: original filename to current directory)")
+	rootCmd.AddCommand(exportCmd)
+}
+
+func runExport(cmd *cobra.Command, args []string) error {
+	client := api.NewClient()
+	resp, err := client.Get("/api/v1/documents/" + args[0] + "/file")
+	if err != nil {
+		fmt.Fprintln(os.Stderr, err)
+		os.Exit(1)
+	}
+	if err := api.CheckError(resp); err != nil {
+		fmt.Fprintln(os.Stderr, err)
+		os.Exit(1)
+	}
+	defer resp.Body.Close()
+
+	outPath, _ := cmd.Flags().GetString("output")
+
+	if outPath == "" {
+		// Try to get filename from Content-Disposition header
+		cd := resp.Header.Get("Content-Disposition")
+		if cd != "" {
+			_, params, err := mime.ParseMediaType(cd)
+			if err == nil && params["filename"] != "" {
+				outPath = params["filename"]
+			}
+		}
+		if outPath == "" {
+			outPath = "document-" + args[0]
+		}
+	}
+
+	if outPath == "-" {
+		_, err := io.Copy(os.Stdout, resp.Body)
+		return err
+	}
+
+	outPath = filepath.Clean(outPath)
+	f, err := os.Create(outPath)
+	if err != nil {
+		return fmt.Errorf("failed to create output file: %w", err)
+	}
+	defer f.Close()
+
+	n, err := io.Copy(f, resp.Body)
+	if err != nil {
+		return fmt.Errorf("failed to write file: %w", err)
+	}
+
+	fmt.Fprintf(os.Stderr, "Saved %s (%d bytes)\n", outPath, n)
+	return nil
+}