config: warn when redis db is set with cluster_addresses

[sloemo01]

Summary

Adds a startup warning for redis.db set alongside redis.cluster_addresses, where it cannot take effect.

Redis Cluster serves only database 0 and rejects SELECT, and go-redis builds cluster clients without a DB field, so the value is dropped and the server connects to database 0. Users in #2129 moved from single-node Redis to a cluster and assumed the setting carried over; nothing told them it could not.

What I checked while looking at that report: the parse error from the 1.2.4 upgrade (field cluster_addresses not found in type config.RedisConfig) came from that tag not having the field yet, so it is unrelated to the current code. On master, cluster mode works: with a real 3-node Redis Cluster (no auth), a room created through one server instance lists and deletes through the other, and pub/sub over the cluster works. The gap left was the dropped db value, which this change surfaces:

WARN  redis configuration would be ignored  {"reason": "redis.db is set to 3 together with redis.cluster_addresses, but Redis Cluster only supports database 0; the setting is ignored"}

A warning instead of a startup error keeps deployments with a leftover db value starting after an upgrade; they were already running against database 0.

Changes

• pkg/config/config.go: RedisConfigWarning() reports the conflict.
• cmd/server/main.go: emits the warning from getConfig, after InitLoggerFromConfig. Output from inside NewConfig is discarded, since the package logger is not initialized until then.
• config-sample.yaml: documents the restriction in the cluster block.
• pkg/config/config_test.go: TestRedisConfigWarning covers single-node with db (no warning), cluster with db 0 (no warning), and cluster with a non-zero db (warns).

Testing

• go build ./cmd/... ./pkg/... clean; go test ./pkg/config/ passes.
• Live check: a server configured with cluster_addresses and db: 3 logs the warning at startup.
• Sabotage check: disabling the condition makes TestRedisConfigWarning/cluster_with_non-zero_db fail; the restored file hash matches.

Refs #2129

[devin-ai-integration]

✅ Devin Review: No Issues Found

Devin Review analyzed this PR and found no bugs or issues to report.