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/docker-compose.yml

@@ -69,7 +69,7 @@ services:
     volumes:
       - ./firebase-credentials.json:/app/firebase-credentials.json:ro
 
-  worker:
+  worker-paid:
     build:
       context: .
       dockerfile: Dockerfile
@@ -78,10 +78,25 @@ services:
       DATABASE_URL: postgres://food_ai:food_ai_local@postgres:5432/food_ai?sslmode=disable
       OPENAI_API_KEY: ${OPENAI_API_KEY}
       KAFKA_BROKERS: kafka:9092
+      WORKER_PLAN: paid
     depends_on:
       postgres:
         condition: service_healthy
-      kafka:
+      kafka-init:
+        condition: service_completed_successfully
+
+  worker-free:
+    build:
+      context: .
+      dockerfile: Dockerfile
+    command: ["./worker"]
+    environment:
+      DATABASE_URL: postgres://food_ai:food_ai_local@postgres:5432/food_ai?sslmode=disable
+      OPENAI_API_KEY: ${OPENAI_API_KEY}
+      KAFKA_BROKERS: kafka:9092
+      WORKER_PLAN: free
+    depends_on:
+      postgres:
         condition: service_healthy
       kafka-init:
         condition: service_completed_successfully