feat(geo): switch geocoder from self-host Photon to OpenCage API

logic_geo проксирует OpenCage (suggest/geocode/reverse): env OPENCAGE_API_KEY,
OPENCAGE_URL, OPENCAGE_TIMEOUT_MS, OPENCAGE_COUNTRYCODE, OPENCAGE_THROTTLE_RPS
(1 rps под free-тариф, 0 - выключить). HTTP 402/403/429 апстрима ->
503 geo_unavailable; source=opencage. Сервис photon и volume photon-data
удалены из swarm compose; eunit-фикстуры под формат OpenCage.

Refs EventHub/EventHubBack#77

docker/.env.example

@@ -43,8 +43,7 @@ REMINDER_LEAD_HOURS=24
 # Uploads (avatar/cover) — каталог на volume /app/data (Back#71)
 UPLOAD_DIR=/app/data/uploads
 UPLOAD_MAX_BYTES=2097152
-# Photon (DevOps#16). Пусто = /v1/geo → 503. Включить: PHOTON_REPLICAS=1 и PHOTON_URL=http://photon:2322
-PHOTON_URL=
-PHOTON_TIMEOUT_MS=3000
-PHOTON_REPLICAS=0
+# OpenCage (DevOps#16). ������ ���� = /v1/geo -> 503 (degrade). Free: 2500/�����, 1 rps.
+OPENCAGE_API_KEY=
+OPENCAGE_TIMEOUT_MS=3000