refactor: split worker into paid/free via WORKER_PLAN env var

Replace dual-consumer priority WorkerPool with a single consumer per
worker process. WORKER_PLAN=paid|free selects the Kafka topic and
consumer group ID (dish-recognition-paid / dish-recognition-free).

docker-compose now runs worker-paid and worker-free as separate services
for independent scaling. Makefile dev target launches both workers locally.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/Makefile

@@ -1,4 +1,4 @@
-.PHONY: run run-worker dev dev-infra-up dev-infra-down test test-integration lint migrate-up migrate-down migrate-create migrate-status docker-up docker-down docker-logs docker-logs-worker
+.PHONY: run run-worker-paid run-worker-free dev dev-infra-up dev-infra-down test test-integration lint migrate-up migrate-down migrate-create migrate-status docker-up docker-down docker-logs docker-logs-worker
 
 ifneq (,$(wildcard .env))
   include .env
@@ -9,14 +9,18 @@ endif
 run:
 	go run ./cmd/server
 
-run-worker:
-	go run ./cmd/worker
+run-worker-paid:
+	WORKER_PLAN=paid go run ./cmd/worker
 
-# Start only infra (postgres, kafka) in Docker, run server + worker locally
+run-worker-free:
+	WORKER_PLAN=free go run ./cmd/worker
+
+# Start only infra (postgres, kafka) in Docker, run server + both workers locally
 dev: dev-infra-up
 	@trap 'kill 0' INT; \
 	go run ./cmd/server & \
-	go run ./cmd/worker & \
+	WORKER_PLAN=paid go run ./cmd/worker & \
+	WORKER_PLAN=free go run ./cmd/worker & \
 	wait
 
 dev-infra-up:
@@ -61,4 +65,4 @@ docker-logs:
 	docker compose logs -f app
 
 docker-logs-worker:
-	docker compose logs -f worker
+	docker compose logs -f worker-paid worker-free