fix(pi3): mask boot-blocking services on the SD image

Boot from cold to root prompt was 2-3 min because Pi OS Trixie waits
on a handful of services that timeout instead of completing:
  - systemd-networkd-wait-online (60s default)
  - NetworkManager-wait-online    (30s default)
  - wpa_supplicant + dhcpcd5      (no usable interfaces)
  - raspi-config / firstboot / userconfig (no point in QEMU)

The SD image was re-baked through scripts/configure-pi3-autologin.sh
with all of them masked (the script grew a `mask_unit` helper that
symlinks each unit to /dev/null inside the rootfs). Login prompt now
appears in ~30s wall.

New manifest version 2026-04-21+autologin+fastboot — same byte count
as the previous build (still 5.4 GiB raw) but a different SHA so the
sidecar-based cache invalidation forces every container to refetch.

backend/app/services/boot_images/manifest.json

@@ -22,13 +22,13 @@
         {
           "name": "raspios-trixie-armhf.img",
           "asset_id": "raspios-trixie-armhf-zst",
-          "sha256": "09bec8e099451d751ee9741e903f694ad641a0c534862efac0caabfc8a2beaf1",
+          "sha256": "36b0e9358940ff1ffa9a0f2da3e74c75a2ff657e67e3ed6780df3c5c0df3038d",
           "size_bytes": 5729419264,
-          "version": "2026-04-21+autologin",
+          "version": "2026-04-21+autologin+fastboot",
           "compressed": {
             "encoding": "zstd",
-            "sha256": "e665edfc94d481f5a756da74f6d494af78917ea4114dc5ad258718dfee1fce5b",
-            "size_bytes": 1488004394
+            "sha256": "8de2ed5e49031a92d56a15e894a9f430949cd21ce5ebe26353839073853162e0",
+            "size_bytes": 1488007643
           }
         }
       ]