fix(pi3): velxio-init reads stdin from /dev/ttyAMA1 (was /dev/console)

User-reported bug: Pi 3 simulator showed boot output but keyboard
input was ignored — the shell was effectively read-only.

Root cause: velxio-init's bash redirect was `</dev/console
>/dev/console`. From userspace, /dev/console is write-only — it is
the kernel's printk target and accepts writes (so we saw boot output
fine) but reads return EOF / block forever. Bash never saw a
keystroke and the user couldn't type.

Fix: read AND write through /dev/ttyAMA1. The 12 s devtmpfs-wait
already in velxio-init guarantees the device node exists by the
time the shell-respawn loop runs. setsid -c still gives bash a
controlling terminal so PS1, job control, and Ctrl-C all work.

Manifest version bumped to 2026-04-21+ttyAMA1; sidecar SHA check
invalidates the cached SD on every velxio backend so the fix lands
without an operator dance.

backend/app/services/boot_images/manifest.json

@@ -22,13 +22,13 @@
         {
           "name": "raspios-trixie-armhf.img",
           "asset_id": "raspios-trixie-armhf-zst",
-          "sha256": "0606578f5a6054004f9466bbeaa99131eacef0c2d3ab7317c2ed22373ce16277",
+          "sha256": "1c5d05449be589fcdbc3283364462d40dd5efbc685e2bd5169fd857a311ff5b0",
           "size_bytes": 5729419264,
-          "version": "2026-04-21+velxio-init-v2",
+          "version": "2026-04-21+ttyAMA1",
           "compressed": {
             "encoding": "zstd",
-            "sha256": "739f4186de775a44f81e4e34a8db2bd76c54ca59e68bd51ca17c36594fa54b4c",
-            "size_bytes": 1488006498
+            "sha256": "2f5e62277cba51bc66f4a2d76666eae34e8ef5a1fbba4169b9f2824830afe115",
+            "size_bytes": 1488004697
           }
         }
       ]