feat(19.3-01): share rate limits across instances through cooler's Redis store

- new module cooler: Install, Handle, OpenRedis (no URL in errors), RateLimitStore
  (atomic fixed-window Lua script with MemoryStore semantics, per-instance fallback)
- serve installs cooler and publishes the Redis surf.Store before BuildRouter;
  BuildRouter looks up a published Store, route:list keeps the memory store
- config keys http.rate_limit.store and redis.url; go-redis v9.23.0
- cooler README, root table row, surf README and rate-limiting, configuration
  and scaling docs

go.mod

@@ -21,6 +21,7 @@ require (
 	github.com/knadh/koanf/v2 v2.3.6
 	github.com/microcosm-cc/bluemonday v1.0.27
 	github.com/nicksnyder/go-i18n/v2 v2.6.1
+	github.com/redis/go-redis/v9 v9.23.0
 	github.com/riverqueue/river v0.47.0
 	github.com/riverqueue/river/riverdriver/riverdatabasesql v0.47.0
 	github.com/riverqueue/river/rivertype v0.47.0
@@ -114,6 +115,7 @@ require (
 	go.opentelemetry.io/otel/sdk v1.44.0 // indirect
 	go.opentelemetry.io/otel/sdk/metric v1.44.0 // indirect
 	go.opentelemetry.io/otel/trace v1.44.0 // indirect
+	go.uber.org/atomic v1.12.0 // indirect
 	go.yaml.in/yaml/v3 v3.0.5 // indirect
 	golang.org/x/sync v0.23.0 // indirect
 	golang.org/x/sys v0.48.0 // indirect