feat: add notification channels for email and webhooks

src/main/java/com/slotline/notification/EmailNotificationChannel.java

@@ -0,0 +1,23 @@
+package com.slotline.notification;
+
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+import org.springframework.stereotype.Component;
+
+@Component
+public class EmailNotificationChannel implements NotificationChannel {
+
+    private static final Logger log = LoggerFactory.getLogger(EmailNotificationChannel.class);
+
+    @Override
+    public String name() {
+        return "email";
+    }
+
+    @Override
+    public void deliver(NotificationMessage message) {
+        log.info("[email] user={} type={} resource='{}' [{} - {}] :: {}",
+                message.recipientUserId(), message.type(), message.resourceName(),
+                message.startTime(), message.endTime(), message.summary());
+    }
+}

src/main/java/com/slotline/notification/HttpWebhookGateway.java

@@ -0,0 +1,38 @@
+package com.slotline.notification;
+
+import com.fasterxml.jackson.databind.ObjectMapper;
+import org.springframework.stereotype.Component;
+
+import java.net.URI;
+import java.net.http.HttpClient;
+import java.net.http.HttpRequest;
+import java.net.http.HttpResponse;
+import java.time.Duration;
+
+@Component
+public class HttpWebhookGateway implements WebhookGateway {
+
+    private final HttpClient httpClient = HttpClient.newBuilder()
+            .connectTimeout(Duration.ofSeconds(5))
+            .build();
+    private final ObjectMapper objectMapper;
+
+    public HttpWebhookGateway(ObjectMapper objectMapper) {
+        this.objectMapper = objectMapper;
+    }
+
+    @Override
+    public void post(String url, NotificationMessage message) {
+        try {
+            String body = objectMapper.writeValueAsString(message);
+            HttpRequest request = HttpRequest.newBuilder(URI.create(url))
+                    .header("Content-Type", "application/json")
+                    .timeout(Duration.ofSeconds(5))
+                    .POST(HttpRequest.BodyPublishers.ofString(body))
+                    .build();
+            httpClient.send(request, HttpResponse.BodyHandlers.discarding());
+        } catch (Exception failure) {
+            throw new WebhookDeliveryException(url, failure);
+        }
+    }
+}

src/main/java/com/slotline/notification/NotificationChannel.java

@@ -0,0 +1,8 @@
+package com.slotline.notification;
+
+public interface NotificationChannel {
+
+    String name();
+
+    void deliver(NotificationMessage message);
+}

src/main/java/com/slotline/notification/NotificationMessage.java

@@ -0,0 +1,14 @@
+package com.slotline.notification;
+
+import java.time.Instant;
+
+public record NotificationMessage(
+        NotificationType type,
+        Long organizationId,
+        Long recipientUserId,
+        Long resourceId,
+        String resourceName,
+        Instant startTime,
+        Instant endTime,
+        String summary) {
+}

src/main/java/com/slotline/notification/NotificationService.java

@@ -0,0 +1,30 @@
+package com.slotline.notification;
+
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+import org.springframework.stereotype.Service;
+
+import java.util.List;
+
+@Service
+public class NotificationService {
+
+    private static final Logger log = LoggerFactory.getLogger(NotificationService.class);
+
+    private final List<NotificationChannel> channels;
+
+    public NotificationService(List<NotificationChannel> channels) {
+        this.channels = channels;
+    }
+
+    public void dispatch(NotificationMessage message) {
+        for (NotificationChannel channel : channels) {
+            try {
+                channel.deliver(message);
+            } catch (RuntimeException failure) {
+                log.warn("Notification channel '{}' failed to deliver {} for user {}",
+                        channel.name(), message.type(), message.recipientUserId(), failure);
+            }
+        }
+    }
+}

src/main/java/com/slotline/notification/NotificationType.java

@@ -0,0 +1,7 @@
+package com.slotline.notification;
+
+public enum NotificationType {
+    RESERVATION_CREATED,
+    RESERVATION_CANCELLED,
+    WAITLIST_PROMOTED
+}

src/main/java/com/slotline/notification/WebhookDeliveryException.java

@@ -0,0 +1,8 @@
+package com.slotline.notification;
+
+public class WebhookDeliveryException extends RuntimeException {
+
+    public WebhookDeliveryException(String url, Throwable cause) {
+        super("Failed to deliver webhook to " + url, cause);
+    }
+}

src/main/java/com/slotline/notification/WebhookGateway.java

@@ -0,0 +1,6 @@
+package com.slotline.notification;
+
+public interface WebhookGateway {
+
+    void post(String url, NotificationMessage message);
+}

src/main/java/com/slotline/notification/WebhookNotificationChannel.java

@@ -0,0 +1,28 @@
+package com.slotline.notification;
+
+import com.slotline.config.SlotlineProperties;
+import org.springframework.boot.autoconfigure.condition.ConditionalOnProperty;
+import org.springframework.stereotype.Component;
+
+@Component
+@ConditionalOnProperty(prefix = "slotline.notifications.webhook", name = "url")
+public class WebhookNotificationChannel implements NotificationChannel {
+
+    private final WebhookGateway gateway;
+    private final String url;
+
+    public WebhookNotificationChannel(WebhookGateway gateway, SlotlineProperties properties) {
+        this.gateway = gateway;
+        this.url = properties.getNotifications().getWebhook().getUrl();
+    }
+
+    @Override
+    public String name() {
+        return "webhook";
+    }
+
+    @Override
+    public void deliver(NotificationMessage message) {
+        gateway.post(url, message);
+    }
+}