Fix: Improve staleness check error handling (bd-2q6d, bd-o4qy, bd-n4td)

Changes:
- CheckStaleness now returns errors for corrupted last_import_time metadata
  instead of silently returning false (bd-o4qy)
- Added handling for empty string metadata (memory store behavior)
- Enhanced warning messages when staleness check fails to be more explicit
  that operation continues with potentially stale data (bd-n4td)
- Added test coverage for corrupted metadata scenario

Closes bd-2q6d, bd-o4qy, bd-n4td

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

cmd/bd/staleness.go

@@ -33,7 +33,8 @@ func ensureDatabaseFresh(ctx context.Context) error {
 	if err != nil {
 		// If we can't determine staleness, allow operation to proceed
 		// (better to show potentially stale data than block user)
-		fmt.Fprintf(os.Stderr, "Warning: could not check database staleness: %v\n", err)
+		fmt.Fprintf(os.Stderr, "Warning: Could not verify database freshness: %v\n", err)
+		fmt.Fprintf(os.Stderr, "Proceeding anyway. Data may be stale.\n\n")
 		return nil
 	}