redis: increase pubsub channel buffer to prevent message drops

[mmatczuk]

The go-redis pubsub.Channel() defaults to a buffer of 100 messages.
Under concurrent writes (e.g. max_in_flight=10 publishing batches of
100), the buffer overflows and messages are silently dropped since
Redis PubSub is fire-and-forget. Increase buffer to 2000.

Fixes CON-411

[claude]

Commits
LGTM

Review
Single-line change increases the go-redis pubsub channel buffer from the default 100 to 2000 via redis.WithChannelSize(2000), preventing silent message drops when the subscriber can't keep up with high-throughput publishers. The fix is well-scoped and the commit message clearly explains the problem and rationale.

LGTM