docs: add payment system installation guide

PAYMENT_INSTALLATION.md

+# Payment System Installation Guide
+
+The AISBF payment system supports both fiat (Stripe/PayPal) and cryptocurrency payments. Cryptocurrency support is optional due to system dependency requirements.
+
+## Installation Options
+
+### Option 1: Core Payment System Only (Fiat Payments)
+
+Install the base requirements without cryptocurrency support:
+
+```bash
+pip install -r requirements.txt
+```
+
+This includes:
+- ✅ Stripe payment processing
+- ✅ PayPal billing agreements
+- ✅ Subscription management
+- ✅ Email notifications
+- ✅ Background scheduler
+- ❌ Cryptocurrency payments (BTC, ETH, USDT, USDC)
+
+### Option 2: Full Payment System (Fiat + Crypto)
+
+#### Step 1: Install System Dependencies
+
+**Ubuntu/Debian:**
+```bash
+sudo apt-get update
+sudo apt-get install -y pkg-config libsecp256k1-dev build-essential
+```
+
+**RHEL/CentOS/Fedora:**
+```bash
+sudo yum install -y pkgconfig libsecp256k1-devel gcc
+```
+
+**Alpine Linux:**
+```bash
+sudo apk add pkgconfig libsecp256k1-dev gcc musl-dev
+```
+
+#### Step 2: Install Python Dependencies
+
+```bash
+pip install -r requirements.txt
+pip install -r requirements-crypto.txt
+```
+
+This includes:
+- ✅ All fiat payment features
+- ✅ Cryptocurrency payments (BTC, ETH, USDT, USDC)
+- ✅ HD wallet generation (BIP32/BIP44)
+- ✅ Blockchain monitoring
+- ✅ Multi-exchange price aggregation
+
+## Troubleshooting
+
+### Error: "Could NOT find PkgConfig"
+
+This means system dependencies are missing. Install them using the commands above for your OS.
+
+### Error: "coincurve build failed"
+
+The `coincurve` package requires `libsecp256k1`. Install system dependencies first, then retry.
+
+### Alternative: Use Pre-built Wheels
+
+If you cannot install system dependencies, try using pre-built wheels:
+
+```bash
+pip install --only-binary :all: coincurve
+pip install -r requirements-crypto.txt
+```
+
+## Configuration
+
+After installation, configure the payment system:
+
+1. Set encryption key for HD wallets:
+   ```bash
+   export ENCRYPTION_KEY="your-fernet-key-here"
+   ```
+
+2. Configure payment gateways in the admin dashboard:
+   - Stripe API keys
+   - PayPal client credentials
+   - Crypto consolidation addresses (if using crypto)
+
+3. Run database migrations:
+   ```bash
+   python main.py  # Migrations run automatically on startup
+   ```
+
+## Features by Installation Type
+
+| Feature | Core Only | Full (Crypto) |
+|---------|-----------|---------------|
+| Stripe Payments | ✅ | ✅ |
+| PayPal Payments | ✅ | ✅ |
+| Subscriptions | ✅ | ✅ |
+| Auto-renewals | ✅ | ✅ |
+| Email Notifications | ✅ | ✅ |
+| Quota Enforcement | ✅ | ✅ |
+| BTC Payments | ❌ | ✅ |
+| ETH/USDT/USDC | ❌ | ✅ |
+| HD Wallet | ❌ | ✅ |
+| Blockchain Monitor | ❌ | ✅ |
+| Wallet Consolidation | ❌ | ✅ |
+
+## Recommendation
+
+For production deployments:
+- **With crypto support**: Install on a server where you have root access to install system dependencies
+- **Without crypto support**: Use the core installation and rely on Stripe/PayPal for all payments
+
+Both options provide a fully functional subscription payment system.