feat: implement performance optimizations and notification plugin system

backend/.env.example

@@ -31,3 +31,24 @@ COMPRESSION_THRESHOLD=1024
 STRIPE_SECRET_KEY=sk_test_...
 STRIPE_PUBLISHABLE_KEY=pk_test_...
 STRIPE_WEBHOOK_SECRET=whsec_...
+
+# Database Read Replicas
+# DB_READ_REPLICA_URLS=postgresql://user:pass@replica1:5432/db,postgresql://user:pass@replica2:5432/db
+# DB_REPLICA_MAX_LAG_MS=5000
+
+# Notification Channels
+# SMTP_HOST=smtp.example.com
+# SMTP_PORT=587
+# SMTP_USER=user@example.com
+# SMTP_PASSWORD=password
+# SMTP_FROM=noreply@agenticpay.com
+# SLACK_WEBHOOK_URL=https://hooks.slack.com/services/YOUR/WEBHOOK/URL
+# SLACK_BOT_TOKEN=xoxb-your-token
+# NOTIFICATION_WEBHOOK_URL=https://your-webhook-endpoint.com/notify
+# NOTIFICATION_WEBHOOK_SECRET=your-webhook-secret
+
+# Redis Cluster
+# REDIS_CLUSTER_NODES=redis1:6379,redis2:6379,redis3:6379
+# REDIS_CLUSTER_MAX_REDIRECTIONS=3
+# REDIS_CLUSTER_RETRY_ATTEMPTS=3
+# REDIS_CLUSTER_RETRY_DELAY=500

backend/docs/NOTIFICATION_CHANNELS.md

@@ -0,0 +1,96 @@
+# Notification Channel Plugin System
+
+## Overview
+
+The notification system uses a channel-based plugin architecture that allows dynamic addition of new notification channels without modifying core code.
+
+## Built-in Channels
+
+- **In-App**: Real-time notifications via WebSocket (highest priority)
+- **Email**: SMTP-based email notifications
+- **Slack**: Slack webhook notifications with rich formatting
+- **Webhook**: Generic webhook for custom integrations
+
+## Creating a Custom Channel
+
+Implement the `NotificationChannel` interface:
+
+```typescript
+import {
+  NotificationChannel,
+  Notification,
+  NotificationDeliveryResult,
+} from "./channel-interface";
+
+export class CustomChannel implements NotificationChannel {
+  readonly id = "custom";
+  readonly name = "Custom Channel";
+  readonly enabled = true;
+  readonly priority = 50;
+
+  async send(notification: Notification): Promise<NotificationDeliveryResult> {
+    // Send notification
+  }
+
+  async format(notification: Notification): Promise<unknown> {
+    // Format for channel
+  }
+
+  async validate(): Promise<boolean> {
+    // Validate configuration
+  }
+
+  async healthCheck(): Promise<boolean> {
+    // Check channel health
+  }
+
+  getRateLimit() {
+    return { maxPerHour: 10, maxPerDay: 50 };
+  }
+}
+```
+
+## Registration
+
+```typescript
+import { channelRegistry } from "./channel-registry";
+import { CustomChannel } from "./channels/custom-channel";
+
+const customChannel = new CustomChannel(config);
+channelRegistry.register(customChannel);
+```
+
+## User Preferences
+
+Users can configure:
+
+- Preferred channels per event type
+- Channel priority/fallback order
+- Quiet hours (no notifications during specified times)
+
+## Fallback Chain
+
+If the primary channel fails, the system automatically tries fallback channels in priority order.
+
+## Rate Limiting
+
+Each channel enforces its own rate limits to prevent spam.
+
+## Environment Variables
+
+```
+# Email Channel
+SMTP_HOST=smtp.example.com
+SMTP_PORT=587
+SMTP_USER=user@example.com
+SMTP_PASSWORD=password
+SMTP_FROM=noreply@agenticpay.com
+
+# Slack Channel
+SLACK_WEBHOOK_URL=https://hooks.slack.com/services/YOUR/WEBHOOK/URL
+SLACK_BOT_TOKEN=xoxb-your-token
+
+# Webhook Channel
+NOTIFICATION_WEBHOOK_URL=https://your-webhook-endpoint.com/notify
+NOTIFICATION_WEBHOOK_SECRET=your-webhook-secret
+```