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/Dockerfile

@@ -4,13 +4,15 @@ WORKDIR /build
 COPY go.mod go.sum ./
 RUN go mod download
 COPY . .
-RUN CGO_ENABLED=0 go build -o /app/server ./cmd/server
+RUN CGO_ENABLED=0 go build -o /app/server ./cmd/server && \
+    CGO_ENABLED=0 go build -o /app/worker ./cmd/worker
 
 # Run
 FROM alpine:3.19
 RUN apk add --no-cache ca-certificates
 WORKDIR /app
 COPY --from=builder /app/server .
+COPY --from=builder /app/worker .
 COPY migrations ./migrations
 EXPOSE 8080
 CMD ["./server"]