Implement ingest-service: MQTT -> ClickHouse (batched) + RabbitMQ event

Subscribes to devices/+/telemetry, validates payloads, buffers writes into
ClickHouse (flush on size or interval), and emits a per-reading event on
RabbitMQ for rule-engine-service to consume later. Adds the initial Postgres
and ClickHouse schema migrations, a Dockerfile, and wires the service into
docker-compose. Verified end-to-end via docker compose + mosquitto_pub: row
lands in ClickHouse and the event reaches the telemetry.new_reading queue.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

services/ingest-service/internal/mqttclient/subscriber.go

@@ -0,0 +1,60 @@
+// Package mqttclient subscribes to device telemetry topics over MQTT.
+package mqttclient
+
+import (
+	"fmt"
+	"log/slog"
+	"time"
+
+	mqtt "github.com/eclipse/paho.mqtt.golang"
+)
+
+// Handler receives one message's raw payload and the time it arrived.
+type Handler func(payload []byte, receivedAt time.Time)
+
+type Config struct {
+	BrokerURL string // e.g. tcp://mosquitto:1883
+	ClientID  string
+	Topic     string // e.g. devices/+/telemetry
+	QoS       byte
+}
+
+type Subscriber struct {
+	client mqtt.Client
+}
+
+// Connect dials the broker and (re-)subscribes to cfg.Topic on every
+// successful connection, so a dropped connection resubscribes automatically
+// once auto-reconnect succeeds.
+func Connect(cfg Config, handler Handler, logger *slog.Logger) (*Subscriber, error) {
+	opts := mqtt.NewClientOptions().
+		AddBroker(cfg.BrokerURL).
+		SetClientID(cfg.ClientID).
+		SetAutoReconnect(true).
+		SetConnectRetry(true).
+		SetOnConnectHandler(func(c mqtt.Client) {
+			token := c.Subscribe(cfg.Topic, cfg.QoS, func(_ mqtt.Client, msg mqtt.Message) {
+				handler(msg.Payload(), time.Now())
+			})
+			token.Wait()
+			if err := token.Error(); err != nil {
+				logger.Error("mqtt subscribe failed", "topic", cfg.Topic, "error", err)
+			}
+		}).
+		SetConnectionLostHandler(func(_ mqtt.Client, err error) {
+			logger.Warn("mqtt connection lost", "error", err)
+		})
+
+	client := mqtt.NewClient(opts)
+	token := client.Connect()
+	token.Wait()
+	if err := token.Error(); err != nil {
+		return nil, fmt.Errorf("connect to mqtt broker %s: %w", cfg.BrokerURL, err)
+	}
+	return &Subscriber{client: client}, nil
+}
+
+// Close disconnects, waiting up to quiesceMS for in-flight work to settle.
+func (s *Subscriber) Close() {
+	s.client.Disconnect(250)
+}