feat: split worker into separate binary (cmd/worker)

Kafka consumers and WorkerPool are moved out of the server process into
a dedicated worker binary. Server now handles HTTP + SSE only; worker
handles Kafka consumption and AI processing.

- cmd/worker/main.go + init.go: new binary with minimal config
  (DATABASE_URL, OPENAI_API_KEY, KAFKA_BROKERS)
- cmd/server: remove WorkerPool, paidConsumer, freeConsumer
- Dockerfile: builds both server and worker binaries
- docker-compose.yml: add worker service
- Makefile: add run-worker and docker-logs-worker targets
- README.md: document worker startup and env vars

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

backend/Makefile

@@ -1,4 +1,4 @@
-.PHONY: run test test-integration lint migrate-up migrate-down migrate-create migrate-status docker-up docker-down docker-logs
+.PHONY: run run-worker 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
@@ -9,6 +9,9 @@ endif
 run:
 	go run ./cmd/server
 
+run-worker:
+	go run ./cmd/worker
+
 # Tests
 test:
 	go test ./... -v -race -count=1
@@ -43,3 +46,6 @@ docker-down:
 
 docker-logs:
 	docker compose logs -f app
+
+docker-logs-worker:
+	docker compose logs -f worker