feat: wire NTP client into firmware build and initialization

- Add ntp.c to CMakeLists.txt SRCS so it's compiled and linked
- Load ntp_server from NVS in load_nvs_config() (default: pool.ntp.org)
- Add ntp_server field to spaxel_state_t
- Initialize NTP after WiFi connects with 10s sync timeout, WARN on failure
- Re-sync NTP after WiFi reconnect (WIFI_LOST state)
- Start periodic 10-minute resync timer via esp_timer
- Add ntp_synced boolean to health JSON message
- Handle ntp_server field in downstream config message
- Fix periodic resync callback to properly stop/restart SNTP

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

firmware/main/CMakeLists.txt

@@ -6,6 +6,7 @@ idf_component_register(
          "ble.c"
          "provision.c"
          "nvs_migration.c"
+         "ntp.c"
     INCLUDE_DIRS "." "${CMAKE_BINARY_DIR}/spaxel-firmware/main"
     REQUIRES esp_wifi esp_netif nvs_flash mdns esp_http_client esp_timer bt driver log esp_http_server mbedtls esp_ota
 )

firmware/main/main.c

@@ -131,6 +131,12 @@ static esp_err_t load_nvs_config(void) {
         g_state.debug = (debug == 1);
     }
 
+    // Load NTP server
+    len = sizeof(g_state.ntp_server);
+    if (nvs_get_str(nvs, NVS_KEY_NTP_SERVER, g_state.ntp_server, &len) != ESP_OK) {
+        strncpy(g_state.ntp_server, "pool.ntp.org", sizeof(g_state.ntp_server));
+    }
+
     nvs_close(nvs);
 
     ESP_LOGI(TAG, "NVS config loaded: provisioned=%d, role=%s, rate=%d Hz",
@@ -200,8 +206,18 @@ static void state_machine_task(void *arg) {
                 if (bits & SPAXEL_EVENT_WIFI_CONNECTED) {
                     ESP_LOGI(TAG, "WiFi connected");
                     wifi_fail_count = 0;
-                    g_state.state = NODE_STATE_MOTHERSHIP_DISCOVERY;
                     discovery_fail_count = 0;
+
+                    // Initialize NTP after WiFi is up
+                    ESP_LOGI(TAG, "Starting NTP sync with server: %s", g_state.ntp_server);
+                    ntp_init();
+                    ntp_start_sync(g_state.ntp_server);
+                    if (!ntp_wait_sync(10000)) {
+                        ESP_LOGW(TAG, "NTP sync failed, proceeding without stagger");
+                    }
+                    ntp_start_periodic_resync();
+
+                    g_state.state = NODE_STATE_MOTHERSHIP_DISCOVERY;
                 } else if (bits & SPAXEL_EVENT_WIFI_FAILED) {
                     wifi_fail_count++;
                     ESP_LOGW(TAG, "WiFi failed (attempt %d)", wifi_fail_count);
@@ -315,6 +331,15 @@ static void state_machine_task(void *arg) {
 
                 if (bits & SPAXEL_EVENT_WIFI_CONNECTED) {
                     ESP_LOGI(TAG, "WiFi reconnected");
+
+                    // Re-sync NTP after reconnect
+                    ntp_init();
+                    ntp_start_sync(g_state.ntp_server);
+                    if (!ntp_wait_sync(10000)) {
+                        ESP_LOGW(TAG, "NTP resync failed after WiFi reconnect");
+                    }
+                    ntp_start_periodic_resync();
+
                     g_state.state = NODE_STATE_MOTHERSHIP_DISCOVERY;
                 } else {
                     wifi_fail_count++;

firmware/main/ntp.c

@@ -35,9 +35,15 @@ static void sntp_sync_time_callback(struct timeval *tv) {
 // Periodic resync timer callback
 static void periodic_resync_callback(void *arg) {
     ESP_LOGI(TAG, "Periodic NTP resync triggered");
+    esp_sntp_stop();
+    s_is_synced = false;
+    if (s_ntp_events) {
+        xEventGroupClearBits(s_ntp_events, NTP_SYNC_BIT);
+    }
+    esp_sntp_setoperatingmode(SNTP_OPMODE_POLL);
     esp_sntp_setservername(0, s_ntp_server);
+    sntp_set_time_sync_notification_cb(sntp_sync_time_callback);
     esp_sntp_init();
-    // No need to wait here - the callback will handle completion
 }
 
 esp_err_t ntp_init(void) {

firmware/main/spaxel.h

@@ -83,6 +83,7 @@ typedef struct {
     bool provisioned;
     bool debug;
     uint8_t mac[6];
+    char ntp_server[64];
     EventGroupHandle_t events;
 } spaxel_state_t;
 

firmware/main/websocket.c

@@ -2,6 +2,7 @@
 #include "spaxel.h"
 #include "csi.h"
 #include "wifi.h"
+#include "ntp.h"
 #include "esp_log.h"
 #include "esp_timer.h"
 #include "esp_system.h"
@@ -348,6 +349,9 @@ esp_err_t websocket_send_health(void) {
         }
     }
 
+    // NTP sync status
+    cJSON_AddBoolToObject(root, "ntp_synced", ntp_is_synced());
+
     char *json = cJSON_PrintUnformatted(root);
     cJSON_Delete(root);
 
@@ -584,6 +588,15 @@ static void handle_config_msg(cJSON *root) {
         g_variance_threshold = (float)var_thresh->valuedouble;
     }
 
+    // NTP server (runtime reconfiguration)
+    cJSON *ntp = cJSON_GetObjectItem(root, "ntp_server");
+    if (ntp && cJSON_IsString(ntp) && strlen(ntp->valuestring) > 0) {
+        strncpy(g_state.ntp_server, ntp->valuestring, sizeof(g_state.ntp_server) - 1);
+        g_state.ntp_server[sizeof(g_state.ntp_server) - 1] = '\0';
+        ESP_LOGI(TAG, "NTP server changed to: %s", g_state.ntp_server);
+        ntp_start_sync(g_state.ntp_server);
+    }
+
     if (changed) {
         csi_set_rate(g_state.packet_rate);
     }