Add Redis Sentinel (HA) support

[MastaP]

Summary

The aggregator's Redis client is currently constructed for a single endpoint only. In HA deployments (ha-compose.yml, sharding-ha-compose.yml) two aggregators share one Redis container, making Redis a single point of failure even though MongoDB and the aggregator itself are HA.

Current state

internal/storage/factory.go:42-51 calls redislib.NewClient(&redislib.Options{Addr: ...}) with a single Host:Port. internal/config/config.go:111-123 only exposes Host, Port, Password, DB, plus pool/timeout fields. There is no SentinelAddrs, MasterName, or cluster node list. No NewFailoverClient / NewClusterClient / NewUniversalClient is called anywhere in the repo.

The library already in use (github.com/redis/go-redis/v9) supports Sentinel and Cluster natively, so the change is small.

Proposed change

Switch the constructor in factory.go to redislib.NewUniversalClient and extend RedisConfig with Sentinel-related fields:

• REDIS_SENTINEL_ADDRS — comma-separated host:port list; when non-empty, Sentinel mode is used.
• REDIS_MASTER_NAME — Sentinel master name (required when REDIS_SENTINEL_ADDRS is set).
• REDIS_SENTINEL_PASSWORD — Sentinel auth password (optional).
• REDIS_SENTINEL_USERNAME — Sentinel ACL username (optional).
• REDIS_ROUTE_BY_LATENCY / REDIS_ROUTE_RANDOMLY — read routing knobs.

UniversalClient keeps the existing single-Addr path working unchanged when no Sentinel addresses are configured, so this is fully backwards compatible.

Acceptance criteria

• RedisConfig exposes the Sentinel fields above, wired through env vars with sensible defaults (empty = disabled).
• factory.go uses NewUniversalClient and constructs the appropriate options based on whether SentinelAddrs is set.
• Existing single-endpoint deployments work without any config change.
• Unit test proves the right client type is selected for each config shape.
• README and changes.txt updated per the project's documentation guidelines.

Out of scope

• Redis Cluster mode (separate concern, can be a follow-up).
• A sentinel-compose.yml example is nice-to-have but not required for this issue.