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/main/gw_client.c

@@ -5,6 +5,7 @@
 #include <string.h>
 
 #include "cJSON.h"
+#include "esp_app_desc.h"
 #include "esp_log.h"
 #include "esp_websocket_client.h"
 
@@ -61,11 +62,17 @@ static void ws_event(void *arg, esp_event_base_t base, int32_t id, void *data)
     (void)base;
     esp_websocket_event_data_t *ev = (esp_websocket_event_data_t *)data;
     switch (id) {
-    case WEBSOCKET_EVENT_CONNECTED:
+    case WEBSOCKET_EVENT_CONNECTED: {
         ESP_LOGI(TAG, "gateway connected");
         s_connected = true;
+        char hello[128];
+        snprintf(hello, sizeof(hello),
+                 "{\"type\":\"hello\",\"device\":\"desklock-p4\",\"fw\":\"%s\"}",
+                 esp_app_get_description()->version);
+        gw_send_text(hello);
         face_set(FACE_IDLE);
         break;
+    }
     case WEBSOCKET_EVENT_DISCONNECTED:
     case WEBSOCKET_EVENT_ERROR:
         if (s_connected) {
@@ -97,6 +104,7 @@ void gw_start(void)
         .buffer_size = 8192,
         .reconnect_timeout_ms = 5000,
         .network_timeout_ms = 10000,
+        .ping_interval_sec = 8, /* keep the link visibly alive through long Tatlock turns */
     };
     s_ws = esp_websocket_client_init(&cfg);
     esp_websocket_register_events(s_ws, WEBSOCKET_EVENT_ANY, ws_event, NULL);