## Summary

Adds a second backend for the realtime runs feed (`useRealtimeRun`,
`subscribeToRunsWithTag`, `subscribeToBatch`), built to stay healthy
when a single busy environment has many subscribers watching many runs
at once. It is gated behind a feature flag with the existing backend as
the default, so nothing changes for users until it is enabled per
environment.

## Design

A run change is published once, as a small self-describing record, to a
single per-environment channel. Every feed is then a predicate over that
one stream rather than owning a channel:

- A per-instance router indexes the currently-held feeds by run, tag,
and batch. When a run changes it hydrates the affected rows once and
serializes them once, then fans the result to every matching feed. One
hot shared tag watched by many subscribers costs a single database query
and serialize, not one per subscriber.
- Feeds that don't match a change are never woken, wake delivery per
environment is coalesced on a leading edge (250ms default) so a burst of
changes costs one wake, and cold reads coalesce onto a single
short-TTL-cached resolve.
- An admission gate bounds how many cold ClickHouse resolves run
concurrently, so a mass reconnect across many distinct filters queues
instead of stampeding the database.
- Changes that land while a client is between long-polls are delivered
on its next poll instead of waiting for the periodic backstop: each
environment buffers its recent change records, subscriptions linger
briefly after the last feed closes, and a newly-armed poll replays
exactly the connection's gap.
- The per-connection replay cursors behind that are shared across
instances via Redis (a single timestamp each), so a poll landing on a
different instance behind the load balancer still reads the connection's
true gap instead of falling back to a cold resolve. Cursor reads have a
bounded deadline and degrade to the cold-read path on any Redis trouble.
- Tag subscriptions with multiple tags match runs carrying all of the
tags, mirroring the existing backend's filter semantics, and live
long-polls hold for about 20 seconds to match its cadence.
- The per-environment channel supports Redis Cluster sharded pub/sub, so
the wake path scales horizontally across shards by environment.
- The backend reports its health through OpenTelemetry metrics (delivery
lag, poll resolution paths, backstop outcomes, replay and cursor-store
activity), with a provisioned Grafana dashboard for local development.

Everything is behind the feature flag and tunable via env vars; the
existing backend remains the default.