feat(docker): add Docker setup, environment examples, and architecture docs

.env.example

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+# 9Router environment contract
+# This file reflects actual runtime usage in the current codebase.
+
+# Required
+JWT_SECRET=change-me-to-a-long-random-secret
+INITIAL_PASSWORD=change-me
+DATA_DIR=/var/lib/9router
+
+# Recommended runtime variables
+PORT=20128
+NODE_ENV=production
+
+# Recommended security and ops variables
+API_KEY_SECRET=endpoint-proxy-api-key-secret
+MACHINE_ID_SALT=endpoint-proxy-salt
+ENABLE_REQUEST_LOGS=false
+
+# Cloud sync variables
+# Must point to this running instance so internal sync jobs can call /api/sync/cloud.
+NEXT_PUBLIC_BASE_URL=http://localhost:20128
+NEXT_PUBLIC_CLOUD_URL=https://9router.com
+
+# Optional outbound proxy variables for upstream provider calls
+# Lowercase variants are also supported: http_proxy, https_proxy, all_proxy, no_proxy
+# HTTP_PROXY=http://127.0.0.1:7890
+# HTTPS_PROXY=http://127.0.0.1:7890
+# ALL_PROXY=socks5://127.0.0.1:7890
+# NO_PROXY=localhost,127.0.0.1
+
+# Currently unused by application runtime (kept as reference)
+# INSTANCE_NAME=9router