feat(notification-service): add Dockerfile, actuator health and alert deduplication

Add multi-stage Dockerfile with eclipse-temurin:17 and HEALTHCHECK on /actuator/health.
Expose /actuator/health endpoint via spring-boot-starter-actuator.
Deduplicate alerts: only notify when a room's CO2 level changes, reset on recovery.

Assisted-by: Claude:claude-sonnet-4-6

notification_service/pom.xml

@@ -37,6 +37,10 @@
       <artifactId>spring-dotenv</artifactId>
       <version>4.0.0</version>
     </dependency>
+    <dependency>
+      <groupId>org.springframework.boot</groupId>
+      <artifactId>spring-boot-starter-actuator</artifactId>
+    </dependency>
 
     <dependency>
       <groupId>org.springframework.boot</groupId>