feat: nora mirror CLI + systemd + install script

nora mirror:
- Pre-fetch dependencies through NORA proxy cache
- npm: --lockfile (v1/v2/v3) and --packages with --all-versions
- pip: requirements.txt parser
- cargo: Cargo.lock parser
- maven: dependency:list output parser
- Concurrent downloads (--concurrency, default 8)
- Progress bar with indicatif
- Health check before start

dist/:
- nora.service — systemd unit with security hardening
- nora.env.example — environment configuration template
- install.sh — automated install (binary + user + systemd + config)

Tested: 103 tests pass, 0 clippy warnings, cargo audit clean.
Smoke: mirrored 70 npm packages from real lockfile in 5.4s.

dist/nora.env.example

@@ -0,0 +1,31 @@
+# NORA configuration — environment variables
+# Copy to /etc/nora/nora.env and adjust
+
+# Server
+NORA_HOST=0.0.0.0
+NORA_PORT=4000
+# NORA_PUBLIC_URL=https://registry.example.com
+
+# Storage
+NORA_STORAGE_PATH=/var/lib/nora
+# NORA_STORAGE_MODE=s3
+# NORA_STORAGE_S3_URL=http://minio:9000
+# NORA_STORAGE_BUCKET=registry
+
+# Auth (optional)
+# NORA_AUTH_ENABLED=true
+# NORA_AUTH_HTPASSWD_FILE=/etc/nora/users.htpasswd
+
+# Rate limiting
+# NORA_RATE_LIMIT_ENABLED=true
+
+# npm proxy
+# NORA_NPM_PROXY=https://registry.npmjs.org
+# NORA_NPM_METADATA_TTL=300
+# NORA_NPM_PROXY_AUTH=user:pass
+
+# PyPI proxy
+# NORA_PYPI_PROXY=https://pypi.org/simple/
+
+# Docker upstreams
+# NORA_DOCKER_UPSTREAMS=https://registry-1.docker.io