feat: deep sleep single-fetch with probabilistic weighting

Refactor timerWakeCycle() to fetch a single random photo per wake
using cycle-mode-aware server-side filtering via Immich's
POST /api/search/random endpoint.

Changes:
- Add fetchOneRandomAssetId() to ImmichClient with support for
  isFavorite, takenAfter, takenBefore filter parameters
- Add timer_last_date NVS field for chronological cursor persistence
- Implement probabilistic filter selection per cycle mode:
  - Random: unfiltered size=1
  - Chronological/ReverseChronological: date cursor in NVS
  - FavoritesWeighted: 66% favorites, 33% unfiltered
  - WeightedChronological: 66% recent (30d), 33% unfiltered
  - WeightedReverseChronological: 66% older (30d), 33% unfiltered
- Add retry loop with filter relaxation on empty results
- Initialize hardware RTC (RX8130CE) firmware-wide:
  - Seed system clock from RTC at boot (immediate time)
  - NTP sync after WiFi connect, write back to RTC
- Add time_utils.h with isoNow(), isoNowMinus30d(), hasValidTime()

Normal mode (PhotoQueue + displayTask) is completely untouched.

Co-authored-by: Cursor <cursoragent@cursor.com>

src/wifi_manager.cpp

@@ -1,6 +1,9 @@
 #include "wifi_manager.h"
 #include <WiFi.h>
 #include <ESPmDNS.h>
+#include <M5Unified.h>
+#include <time.h>
+#include <sys/time.h>
 #include "config.h"
 
 void WiFiManager::begin(SettingsManager& settings) {
@@ -57,6 +60,7 @@ void WiFiManager::startStation() {
             Serial.printf("[wifi] Connected! IP: %s, RSSI: %d\n",
                           WiFi.localIP().toString().c_str(), WiFi.RSSI());
             WiFi.setSleep(WIFI_PS_MIN_MODEM);
+            syncNTP();
             setupMDNS("papercolor");
             return;
         }
@@ -88,3 +92,37 @@ bool WiFiManager::attemptConnection(const String& ssid, const String& pass) {
     }
     return true;
 }
+
+void WiFiManager::syncNTP() {
+    configTime(0, 0, "pool.ntp.org", "time.nist.gov");
+
+    // Wait up to 5 seconds for NTP sync
+    unsigned long start = millis();
+    time_t now = 0;
+    while (now < 1600000000 && (millis() - start) < 5000) {
+        delay(100);
+        time(&now);
+    }
+
+    if (now < 1600000000) {
+        Serial.println("[wifi] NTP sync timed out");
+        return;
+    }
+
+    // Write synced time to hardware RTC
+    struct tm t;
+    gmtime_r(&now, &t);
+
+    m5::rtc_datetime_t dt;
+    dt.date.year = t.tm_year + 1900;
+    dt.date.month = t.tm_mon + 1;
+    dt.date.date = t.tm_mday;
+    dt.time.hours = t.tm_hour;
+    dt.time.minutes = t.tm_min;
+    dt.time.seconds = t.tm_sec;
+    M5.Rtc.setDateTime(dt);
+
+    Serial.printf("[wifi] NTP synced: %04d-%02d-%02d %02d:%02d:%02d UTC, written to RTC\n",
+                  dt.date.year, dt.date.month, dt.date.date,
+                  dt.time.hours, dt.time.minutes, dt.time.seconds);
+}