Feature/rate limiting implementation

[LaGodxy]

✅ Rate Limiting Implementation Complete

Features Implemented:

• Redis-backed rate limiting using @nestjs/throttler
• Configurable limits via environment variables
• Stricter limits for auth endpoints: 5 requests per 15 minutes per IP
• Invoice creation limits: 20 requests per hour per user
• 429 responses with Retry-After headers
• E2E tests for rate limiting functionality

Files Created/Modified:

• src/config/throttler.config.ts - Rate limiting configuration
• src/throttler/throttler.module.ts - Redis-backed throttler module
• src/throttler/throttler-storage-redis.service.ts - Custom Redis storage
• src/auth/auth.controller.ts - Applied rate limiting to auth endpoints
• src/invoices/invoices.controller.ts - Applied rate limiting to invoice creation
• test/rate-limiting.e2e-spec.ts - Comprehensive E2E tests
• .github/workflows/backend.yml - Added Redis service for CI
• src/app.module.ts - Integrated throttler module and config

Environment Variables Added:

# General rate limiting
THROTTLE_TTL=60
THROTTLE_LIMIT=100

# Auth endpoints (stricter)
THROTTLE_AUTH_TTL=900  # 15 minutes
THROTTLE_AUTH_LIMIT=5   # 5 attempts

# Invoice creation
THROTTLE_INVOICE_TTL=3600  # 1 hour  
THROTTLE_INVOICE_LIMIT=20   # 20 invoices

# Redis configuration
REDIS_HOST=localhost
REDIS_PORT=6379
REDIS_PASSWORD=
REDIS_DB=0
REDIS_KEY_PREFIX=invoisio:throttle:

closes #103

[Cedarich]

Please fix E2E Test