Add .env.example documenting environment variables

.env.example

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+# Docker registry URL (used in docker-compose.yml)
+REGISTRY_URL=registry.kucharczyk.xyz
+
+# Container timezone
+TZ=Europe/Prague
+
+# User/group IDs for container (used in entrypoint.sh)
+PUID=1000
+PGID=100
+
+# External port mapping
+TIMETRACKER_EXTERNAL_PORT=8000
+
+# Data storage path
+DOCKER_STORAGE_PATH=/tmp
+
+# Django production mode (set to "1" for production)
+PROD=1
+
+# Database directory (overrides default in settings.py)
+DATA_DIR=/home/timetracker/data
+
+# CSRF trusted origins
+CSRF_TRUSTED_ORIGINS=https://tracker.kucharczyk.xyz