fix: Docker self-update shows pull instructions instead of silently failing

The self-updater extracted files inside the container but Docker restarts
from the original image, discarding all changes. Now detects Docker via
/.dockerenv and returns pull commands for the user to run on their host.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

backend/main.py

@@ -8135,6 +8135,9 @@ async def system_update(request: Request):
             status_code=500,
             media_type="application/json",
         )
+    # Docker: skip restart — user must pull new images manually
+    if result.get("status") == "docker":
+        return result
     # Schedule restart AFTER response flushes (2s delay)
     threading.Timer(2.0, schedule_restart, args=[project_root]).start()
     return result