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/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()
+}