feat: async dish recognition (Kafka/Watermill/SSE) + remove Wire + consolidate migrations

Async recognition pipeline:
- POST /ai/recognize-dish → 202 {job_id, queue_position, estimated_seconds}
- GET /ai/jobs/{id}/stream — SSE stream: queued → processing → done/failed
- Kafka topics: ai.recognize.paid (3 partitions) + ai.recognize.free (1 partition)
- 5-worker WorkerPool with priority loop (paid consumers first)
- SSEBroker via PostgreSQL LISTEN/NOTIFY
- Kafka adapter migrated from franz-go to Watermill (watermill-kafka/v2)
- Docker Compose: added Kafka + Zookeeper + kafka-init service
- Flutter: recognition_service.dart uses SSE; home_screen shows live job status

Remove google/wire (archived):
- Deleted wire.go (wireinject spec) and wire_gen.go
- Added cmd/server/init.go — plain Go manual DI, same initApp() logic
- Removed github.com/google/wire from go.mod

Consolidate migrations:
- Merged 001_initial_schema + 002_seed_data + 003_recognition_jobs into single 001_initial_schema.sql
- Deleted 002_seed_data.sql and 003_recognition_jobs.sql

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

backend/internal/adapters/kafka/consumer.go

@@ -0,0 +1,66 @@
+package kafka
+
+import (
+	"context"
+	"log/slog"
+
+	"github.com/ThreeDotsLabs/watermill"
+	wmkafka "github.com/ThreeDotsLabs/watermill-kafka/v2/pkg/kafka"
+)
+
+// Consumer wraps a Watermill Kafka subscriber for consuming a single topic within a consumer group.
+type Consumer struct {
+	subscriber *wmkafka.Subscriber
+	topic      string
+}
+
+// NewConsumer creates a Consumer subscribed to the given topic within a consumer group.
+func NewConsumer(brokers []string, groupID, topic string) (*Consumer, error) {
+	subscriber, createError := wmkafka.NewSubscriber(
+		wmkafka.SubscriberConfig{
+			Brokers:               brokers,
+			ConsumerGroup:         groupID,
+			Unmarshaler:           wmkafka.DefaultMarshaler{},
+			OverwriteSaramaConfig: wmkafka.DefaultSaramaSubscriberConfig(),
+		},
+		watermill.NopLogger{},
+	)
+	if createError != nil {
+		return nil, createError
+	}
+	return &Consumer{subscriber: subscriber, topic: topic}, nil
+}
+
+// Run subscribes to the Kafka topic and writes job IDs to the out channel until runContext is cancelled.
+// Call this in a dedicated goroutine — it blocks until the context is done.
+// Each message is Ack'd after its job ID is successfully forwarded to the channel,
+// or Nack'd when the context is cancelled before forwarding completes.
+func (consumer *Consumer) Run(runContext context.Context, out chan<- string) {
+	messageChannel, subscribeError := consumer.subscriber.Subscribe(runContext, consumer.topic)
+	if subscribeError != nil {
+		slog.Error("kafka consumer subscribe", "topic", consumer.topic, "err", subscribeError)
+		return
+	}
+	for {
+		select {
+		case msg, ok := <-messageChannel:
+			if !ok {
+				return
+			}
+			select {
+			case out <- string(msg.Payload):
+				msg.Ack()
+			case <-runContext.Done():
+				msg.Nack()
+				return
+			}
+		case <-runContext.Done():
+			return
+		}
+	}
+}
+
+// Close shuts down the underlying Kafka subscriber.
+func (consumer *Consumer) Close() {
+	_ = consumer.subscriber.Close()
+}

backend/internal/adapters/kafka/producer.go

@@ -0,0 +1,42 @@
+package kafka
+
+import (
+	"context"
+
+	"github.com/ThreeDotsLabs/watermill"
+	"github.com/ThreeDotsLabs/watermill/message"
+	wmkafka "github.com/ThreeDotsLabs/watermill-kafka/v2/pkg/kafka"
+)
+
+// Producer wraps a Watermill Kafka publisher for publishing messages to Kafka topics.
+type Producer struct {
+	publisher message.Publisher
+}
+
+// NewProducer creates a Producer connected to the given brokers.
+func NewProducer(brokers []string) (*Producer, error) {
+	publisher, createError := wmkafka.NewPublisher(
+		wmkafka.PublisherConfig{
+			Brokers:   brokers,
+			Marshaler: wmkafka.DefaultMarshaler{},
+		},
+		watermill.NopLogger{},
+	)
+	if createError != nil {
+		return nil, createError
+	}
+	return &Producer{publisher: publisher}, nil
+}
+
+// Publish writes a single message to the named topic.
+// The context parameter is accepted for interface compatibility but is not forwarded
+// to the Watermill publisher, which does not accept a context.
+func (producer *Producer) Publish(_ context.Context, topic, jobID string) error {
+	msg := message.NewMessage(watermill.NewUUID(), []byte(jobID))
+	return producer.publisher.Publish(topic, msg)
+}
+
+// Close shuts down the underlying Kafka publisher.
+func (producer *Producer) Close() {
+	_ = producer.publisher.Close()
+}