feat(push-gateway,msg): redis pubsub backplane for real-time notifications

M7: ADR-040 Redis Pub/Sub as state routing backplane

- push-gateway: remove Kafka consumer, add SSE endpoint

- SSE: subscribe to Redis user:{userId}:notify on connect

- msg: publish notifications to Redis Pub/Sub instead of HTTP push

- docker-compose: remove Kafka env from push-gateway

services/push-gateway/go.mod

@@ -10,7 +10,6 @@ require (
 	github.com/gorilla/websocket v1.5.3
 	github.com/prometheus/client_golang v1.23.2
 	github.com/redis/go-redis/v9 v9.7.0
-	github.com/segmentio/kafka-go v0.4.48
 	go.opentelemetry.io/contrib/instrumentation/github.com/gin-gonic/gin/otelgin v0.69.0
 	go.opentelemetry.io/otel/trace v1.44.0
 )
@@ -35,7 +34,6 @@ require (
 	github.com/goccy/go-yaml v1.19.2 // indirect
 	github.com/grpc-ecosystem/grpc-gateway/v2 v2.29.0 // indirect
 	github.com/json-iterator/go v1.1.12 // indirect
-	github.com/klauspost/compress v1.18.0 // indirect
 	github.com/klauspost/cpuid/v2 v2.3.0 // indirect
 	github.com/leodido/go-urn v1.4.0 // indirect
 	github.com/mattn/go-isatty v0.0.22 // indirect
@@ -43,7 +41,6 @@ require (
 	github.com/modern-go/reflect2 v1.0.2 // indirect
 	github.com/munnerz/goautoneg v0.0.0-20191010083416-a7dc8b61c822 // indirect
 	github.com/pelletier/go-toml/v2 v2.3.1 // indirect
-	github.com/pierrec/lz4/v4 v4.1.15 // indirect
 	github.com/prometheus/client_model v0.6.2 // indirect
 	github.com/prometheus/common v0.66.1 // indirect
 	github.com/prometheus/procfs v0.16.1 // indirect