chore: add CHARON_ENCRYPTION_KEY to all Docker Compose files and README

- Add encryption key environment variable to docker-compose.yml,
  docker-compose.dev.yml, docker-compose.local.yml, docker-compose.test.yml
- Update README.md Quick Start examples (compose and docker run)
- Include generation instructions: openssl rand -base64 32

Required for DNS provider and plugin features which encrypt sensitive data at rest.

.docker/compose/docker-compose.local.yml

@@ -13,6 +13,8 @@ services:
       - CHARON_ENV=development
       - CHARON_DEBUG=1
       - TZ=America/New_York
+      # Generate with: openssl rand -base64 32
+      - CHARON_ENCRYPTION_KEY=your-32-byte-base64-key-here
       - CHARON_HTTP_PORT=8080
       - CHARON_DB_PATH=/app/data/charon.db
       - CHARON_FRONTEND_DIR=/app/frontend/dist