build: make the receiver service headless-console ready

The receiver does not need a window manager: SDL3's KMSDRM backend
renders straight to the kernel display pipeline, which is the right
setup for a small-board appliance. But a systemd system service has
no controlling terminal, and SDL's KMSDRM backend expects one for
its VT handling — so the headless path would have failed at boot.
Give the unit StandardInput=tty with TTYPath=/dev/tty1 (harmless in
desktop mode), and document the headless recipe in the unit, the
install script output, and the RUNBOOK: Raspberry Pi OS Lite with
Console Autologin, no desktop enabled (a compositor would hold the
DRM master), and consoleblank=0.

systemd/screencast-receiver.service

@@ -27,6 +27,11 @@ StartLimitBurst=10
 Type=simple
 User=__SC_RECEIVER_USER__
 ExecStart=__SC_RECEIVER_BIN__ --receive
+# SDL's KMSDRM backend (headless console) expects a controlling terminal
+# for its VT handling; system services have none by default. Harmless in
+# desktop mode, where the process never touches the tty.
+StandardInput=tty
+TTYPath=/dev/tty1
 # SIGTERM (systemctl stop) shuts the receiver down gracefully, which
 # withdraws its mDNS announcement. Never SIGKILL it; the systemd default
 # KillMode escalates only after the graceful timeout.
@@ -43,4 +48,12 @@ PrivateTmp=true
 #Environment=DISPLAY=:0
 
 [Install]
-WantedBy=multi-user.target
+WantedBy=multi-user.target
+
+# Headless console recipe (no window manager needed — SDL renders directly
+# to the kernel via KMSDRM):
+#   * Raspberry Pi OS Lite, no desktop enabled (a running compositor would
+#     hold the DRM master and the receiver could not take it)
+#   * raspi-config → System Options → Boot → Console Autologin
+#   * disable console blanking: add consoleblank=0 to
+#     /boot/firmware/cmdline.txt