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/time_utils.h

@@ -0,0 +1,33 @@
+#pragma once
+
+#include <Arduino.h>
+#include <time.h>
+
+// Returns true if the system clock has been set to a plausible time (year >= 2020).
+inline bool hasValidTime() {
+    time_t now = time(nullptr);
+    struct tm t;
+    gmtime_r(&now, &t);
+    return (t.tm_year + 1900) >= 2020;
+}
+
+// Returns the current UTC time as an ISO 8601 string (e.g., "2026-08-04T17:30:00.000Z").
+inline String isoNow() {
+    time_t now = time(nullptr);
+    struct tm t;
+    gmtime_r(&now, &t);
+    char buf[32];
+    strftime(buf, sizeof(buf), "%Y-%m-%dT%H:%M:%S.000Z", &t);
+    return String(buf);
+}
+
+// Returns (now - 30 days) as an ISO 8601 UTC string.
+inline String isoNowMinus30d() {
+    time_t now = time(nullptr);
+    now -= 30 * 24 * 3600;  // Subtract 30 days in seconds
+    struct tm t;
+    gmtime_r(&now, &t);
+    char buf[32];
+    strftime(buf, sizeof(buf), "%Y-%m-%dT%H:%M:%S.000Z", &t);
+    return String(buf);
+}