feat(nixos): enable Magic SysRq keys for safe crash recovery

modules/nixos/default.nix

@@ -89,6 +89,10 @@ in
     # genuine ZFS-root downstream can still force it back on.
     boot.zfs.forceImportRoot = lib.mkDefault false;
 
+    # Magic SysRq Keys: safety net for Wayland lockups. Alt+SysRq+REISUB allows
+    # safe reboot without data loss when the compositor hangs.
+    boot.kernel.sysctl."kernel.sysrq" = lib.mkDefault 1;
+
     # ── Wayland session: Hyprland ────────────────────────────────────
     # Installs the binary, registers the session, wires up
     # xdg-desktop-portal-hyprland. Configuration is Home Manager's job.
@@ -242,6 +246,13 @@ in
         };
       };
 
+    # Background filesystem health checks. Scans all BTRFS filesystems to detect
+    # and (if RAID/dup) correct bitrot before it propagates.
+    services.btrfs.autoScrub = {
+      enable = lib.mkDefault true;
+      interval = lib.mkDefault "monthly";
+    };
+
     # ── Fonts ────────────────────────────────────────────────────────
     # The ten most popular Nerd Fonts ship by default, so any of them
     # can be named in the theme state's fonts.mono and actually resolve