Add Raspberry Pi setup guide

docs/RASPBERRY_PI.md

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+# Raspberry Pi 3B setup
+
+Recommended OS:
+
+```text
+Raspberry Pi OS Lite 32-bit
+```
+
+## Base packages
+
+```bash
+sudo apt update
+sudo apt upgrade -y
+sudo apt install -y git curl sqlite3 ufw fail2ban rsync
+```
+
+## Node.js
+
+Use Node.js 20 LTS where possible. If the Pi 3B feels too tight on RAM, Node.js 18 LTS can be used for MVP.
+
+## Clone project
+
+```bash
+git clone https://github.com/viktor138irk/chat.git
+cd chat
+npm install
+```
+
+## Backend config
+
+```bash
+cp backend/.env.example backend/.env
+nano backend/.env
+```
+
+Important values:
+
+```env
+APP_HOST=0.0.0.0
+APP_PORT=3000
+DATABASE_PATH=./data/chat.sqlite
+PUBLIC_API_URL=https://api.example.ru
+PUBLIC_WS_URL=wss://api.example.ru/ws
+```
+
+## Run backend
+
+Development:
+
+```bash
+npm run dev:backend
+```
+
+Production with PM2:
+
+```bash
+sudo npm install -g pm2
+pm2 start backend/src/server.js --name raspi-chat-backend
+pm2 save
+pm2 startup
+```
+
+## Network model
+
+Preferred production model:
+
+```text
+VPS public internet
+  -> WireGuard
+  -> Raspberry Pi backend on 10.8.0.2:3000
+```
+
+Do not expose Node.js port 3000 directly to the public internet.