feat: implement performance optimizations and notification plugin system

[Georgechisom]

Summary

This PR implements four major improvements to enhance performance, scalability, and extensibility of the AgenticPay platform.

Changes

1. Frontend Route-Based Code Splitting with Predictive Preload (Issue #498)

Implemented granular code splitting with intelligent preloading to reduce initial bundle size and improve route transition speed.

Key Features:

• Route-specific chunks targeting sub-50KB sizes
• Intersection Observer for viewport-based preloading
• Hover and touch-start preloading for instant navigation
• Predictive preloading based on common navigation patterns
• Data-saver mode detection to conserve bandwidth
• PrefetchLink component with configurable priority

Files:

• frontend/lib/preload.ts - Core preloading utility
• frontend/components/common/link-prefetcher.tsx - React component wrapper
• frontend/next.config.ts - Enhanced webpack configuration

2. Database Read Replicas with Automatic Query Routing (Issue #497)

Implemented read replica support to distribute read-heavy workloads and reduce primary database contention.

Key Features:

• Automatic query classification (read vs write)
• Round-robin replica selection with health checks
• Replication lag monitoring with configurable thresholds
• Session stickiness for read-after-write consistency
• Automatic failover to primary when replicas unavailable
• Prisma middleware for transparent routing

Files:

• backend/src/db/replica-router.ts - Query routing logic
• backend/src/config/database.ts - Replica configuration

3. Channel-Based Notification Plugin System (Issue #495)

Refactored notification delivery into a plugin-based architecture for easy extensibility.

Key Features:

• Common NotificationChannel interface for all channels
• Built-in channels: Email, Slack, In-App, Webhook
• Dynamic channel registry with auto-discovery
• User notification preferences per event type
• Fallback chain when primary channel fails
• Per-channel rate limiting
• Quiet hours support
• Channel-specific formatting (HTML for email, markdown for Slack)

Files:

• backend/src/notifications/channel-interface.ts - Core interfaces
• backend/src/notifications/channel-registry.ts - Plugin registry
• backend/src/notifications/dispatcher.ts - Routing and fallback logic
• backend/src/notifications/channels/ - Channel implementations
• backend/docs/NOTIFICATION_CHANNELS.md - Developer documentation

4. Redis Cluster Mode for Horizontal Scaling (Issue #499)

Added Redis Cluster support to enable horizontal scaling and eliminate single-instance bottlenecks.

Key Features:

• Consistent hashing with CRC16 slot calculation
• Transparent MOVED/ASK redirection handling
• Automatic slot map updates
• Cluster health monitoring with memory tracking
• Pipeline operations within same slot
• Graceful degradation on node failures
• Support for hashtag-based key grouping

Files:

• backend/src/cache/cluster-client.ts - Cluster client wrapper

Configuration

Added environment variables to backend/.env.example:

# Database Read Replicas
DB_READ_REPLICA_URLS=postgresql://user:pass@replica1:5432/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
NOTIFICATION_WEBHOOK_URL=https://your-webhook-endpoint.com/notify

# Redis Cluster
REDIS_CLUSTER_NODES=redis1:6379,redis2:6379,redis3:6379
REDIS_CLUSTER_MAX_REDIRECTIONS=3

Testing

Tests are left for future implementation as requested. Manual testing recommended for:

• Route preloading behavior in browser DevTools
• Replica query routing with replication lag scenarios
• Notification delivery across different channels
• Redis cluster failover and slot redistribution

Migration Notes

• Redis Cluster requires infrastructure changes (ElastiCache Cluster or self-hosted cluster setup)
• Read replicas require PostgreSQL replication configuration
• Notification channels require service credentials in environment variables
• Frontend code splitting is backward compatible

Performance Targets

• Initial JS bundle: target sub-150KB
• Route chunks: target sub-50KB each
• Database read throughput: linear scaling with replica count
• Redis throughput: linear scaling with cluster nodes
• Notification delivery: sub-100ms for in-app, sub-5s for external channels

Closes #498
Closes #497
Closes #495
Closes #499

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