feat: add Redis adapter for distributed rate limiting and notification cache

[deannos]

Overview

Redis serves two roles:

1. Rate limiting — replace per-process in-memory limiter with a shared Redis counter so limits work correctly across multiple server instances
2. Notification cache — cache recent notifications per user to skip DB reads on hot list endpoints

Motivation

The current in-memory rate limiter resets on every instance restart and does not share state across replicas. Under horizontal scaling, each replica has its own limit — a client can exceed the intended rate by hitting different instances.

Work Items

Rate Limiting

• middleware/redis_rate_limit.go — RedisRateLimit() using go-redis/redis_rate
• Replace ipRateLimiter middleware in router/router.go

Notification Cache

• storage/redis/notification_cache.go — read-through/write-through cache wrapping NotificationRepository
• Cache invalidation on MarkRead, DeleteNotification, SendNotification

Config additions

REDIS_ADDR=redis:6379
REDIS_PASSWORD=
REDIS_DB=0
REDIS_CACHE_TTL=60s

Wire-up

rdb := redis.NewClient(&redis.Options{Addr: cfg.RedisAddr})
notifRepo := sqlite.NewNotificationRepo(database)
cachedRepo := rediscache.NewNotificationCache(notifRepo, rdb, cfg.RedisCacheTTL)
// pass cachedRepo to NotificationService

Depends on

#20 (repository interfaces), #21 (service layer)

Ref: docs/enhancementv1.md — Future Integration: Redis

[deannos]

Repository interfaces in place. Redis integration has two parts: (1) middleware/redis_rate_limit.go for multi-instance rate limiting; (2) storage/redis/ cache wrapping NotificationRepository for hot list reads. Implementation deferred.