Device registry (/devices + hello), static-IP fallback, WS keepalive, uncompressed fonts

- gateway: GET /devices registry (per-IP connect count, timestamps,
  device/fw from new 'hello' protocol event); graceful utterance
  errors -> error event; uvicorn ws-ping-timeout 120s
- firmware: hello on WS connect; static 192.168.86.53 fallback after
  15s without DHCP; scan-on-boot removed (kept for diagnostics);
  ping_interval 8s; fonts regenerated --no-compress (compressed
  glyphs render blank with LV_USE_FONT_COMPRESSED off); 8MB factory
  partition; montserrat_28; LVGL lock timeout semantics fixed
- docs: sauron endpoint plan, power ladder, versioning policy

Known issue: 'Outside' AP association succeeds but no L2 traffic
flows (no DHCP, no ARP) — worked once at 22:05 then never again;
ToJ side verified clean. Investigation ongoing.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

firmware/sdkconfig.defaults

@@ -28,3 +28,6 @@ CONFIG_LV_FONT_MONTSERRAT_28=y
 
 # ESP-Hosted C6 radio
 CONFIG_ESP_WIFI_SOFTAP_SUPPORT=n
+
+# custom fonts are uncompressed, but enable the decoder as belt-and-braces
+CONFIG_LV_USE_FONT_COMPRESSED=y