Switch doorbell-viewport to cold start, add backlight diagnostics

- prebuffer_mode=cold: mpv starts on demand at ring/touch event
- Add tty group to service user for DRM TTY access
- Log backlight brightness changes for diagnostics
- Remove --really-quiet suppression during debug (now restored)

2 files changed, 4 insertions, 3 deletions

diff --git a/doorbell-viewport/files/doorbell-viewport.py b/doorbell-viewport/files/doorbell-viewport.py
index 2c5b50a..74241e3 100644
--- a/doorbell-viewport/files/doorbell-viewport.py
+++ b/doorbell-viewport/files/doorbell-viewport.py
@@ -145,16 +145,17 @@ class DisplayController:
             log.warning("No backlight device found in /sys/class/backlight")
             return
         path = paths[0]
-        log.debug("Backlight device: %s", path)
         if enabled:
             try:
                 max_b = int((path / "max_brightness").read_text().strip())
                 (path / "brightness").write_text(str(max_b))
+                log.info("Backlight %s: brightness -> %d (max)", path.name, max_b)
             except Exception as exc:
                 log.error("Backlight on failed: %s", exc)
         else:
             try:
                 (path / "brightness").write_text("0")
+                log.info("Backlight %s: brightness -> 0", path.name)
             except Exception as exc:
                 log.error("Backlight off failed: %s", exc)
 
@@ -403,7 +404,7 @@ class DoorbellViewport:
             self.mpv_proc = await asyncio.create_subprocess_exec(
                 *cmd,
                 stdout=subprocess.DEVNULL,
-                stderr=subprocess.DEVNULL,
+                stderr=None,  # pass through to journal
             )
             log.info("mpv started (pid=%d)", self.mpv_proc.pid)
             asyncio.create_task(self._watch_mpv())
diff --git a/doorbell-viewport/tasks/main.yaml b/doorbell-viewport/tasks/main.yaml
index fa2b4da..fd395c7 100644
--- a/doorbell-viewport/tasks/main.yaml
+++ b/doorbell-viewport/tasks/main.yaml
@@ -7,7 +7,7 @@
     shell: "/bin/false"
     system: "yes"
     create_home: "no"
-    groups: "video,input,render"
+    groups: "video,input,render,tty"
     append: "yes"
 
 - name: allow doorbell-viewport user to control sysfs backlight