feat(manager): add remote VM support via NEXUS_SERVER_HOST

- config.py: add NEXUS_SERVER_HOST env var for cross-VM deployment
- health_service.py: graceful Redis failure when only localhost-bound
- systemd service: document remote VM configuration
- README: add dedicated 'VM separada' installation section
- .env.example: new file with remote connection examples

manager/.env.example

@@ -0,0 +1,25 @@
+# Nexus Instance Manager — Environment Variables
+# Copy to .env and fill in your values.
+
+# ─── Database (REQUIRED) ───────────────────────────────────────────────────
+# If manager runs on a separate VM, use the IP of the PostgreSQL server.
+MASTER_DB_URL=postgresql://nexus:PASSWORD@192.168.10.91/nexus_autopartes
+TENANT_DB_URL_TEMPLATE=postgresql://nexus:PASSWORD@192.168.10.91/{db_name}
+
+# ─── Remote Nexus Server IP (for VM-separated deployment) ──────────────────
+# IP or hostname of the server running POS, Dashboard, Quart, Redis.
+NEXUS_SERVER_HOST=192.168.10.91
+
+# ─── Security (REQUIRED) ───────────────────────────────────────────────────
+MANAGER_JWT_SECRET=change-me-to-a-random-64-char-hex-string
+
+# ─── Demo Defaults ─────────────────────────────────────────────────────────
+DEMO_DEFAULT_DAYS=14
+DEMO_DEFAULT_PIN=0000
+
+# ─── Redis (OPTIONAL — health check only) ──────────────────────────────────
+# Redis may only listen on localhost. If so, health check will show warning.
+REDIS_URL=redis://192.168.10.91:6379/0
+
+# ─── Internal ──────────────────────────────────────────────────────────────
+POS_DIR=/home/Autopartes/pos