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/cmd/server/app.go

@@ -9,9 +9,8 @@ import (
 
 // App bundles the HTTP handler with background services that need lifecycle management.
 type App struct {
-	handler    http.Handler
-	workerPool *recognition.WorkerPool
-	sseBroker  *recognition.SSEBroker
+	handler   http.Handler
+	sseBroker *recognition.SSEBroker
 }
 
 // ServeHTTP implements http.Handler.
@@ -19,9 +18,8 @@ func (application *App) ServeHTTP(responseWriter http.ResponseWriter, request *h
 	application.handler.ServeHTTP(responseWriter, request)
 }
 
-// Start launches the SSE broker's LISTEN loop and the worker pool goroutines.
+// Start launches the SSE broker's LISTEN loop.
 // Call this once before the HTTP server begins accepting connections.
 func (application *App) Start(applicationContext context.Context) {
 	application.sseBroker.Start(applicationContext)
-	application.workerPool.Start(applicationContext)
 }