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/settings.cpp

@@ -29,6 +29,7 @@ Settings SettingsManager::get() {
     s.show_battery  = readU8("show_batt", 1);
     s.queue_cursor  = readU32("queue_cursor", 0);
     s.albums_json   = readString("albums_json", "[]");
+    s.timer_last_date = readString("timer_date", "");
     return s;
 }
 
@@ -51,6 +52,7 @@ void SettingsManager::save(const Settings& s) {
     writeU8("show_batt", s.show_battery);
     writeU32("queue_cursor", s.queue_cursor);
     writeString("albums_json", s.albums_json);
+    writeString("timer_date", s.timer_last_date);
     Serial.println("[settings] All settings saved");
 }