feat(subscription): add SkillSubscriptionService domain service

server/skillhub-domain/src/main/java/com/iflytek/skillhub/domain/social/SkillSubscriptionService.java

@@ -0,0 +1,63 @@
+package com.iflytek.skillhub.domain.social;
+
+import com.iflytek.skillhub.domain.shared.exception.DomainNotFoundException;
+import com.iflytek.skillhub.domain.skill.SkillRepository;
+import com.iflytek.skillhub.domain.social.event.SkillSubscribedEvent;
+import com.iflytek.skillhub.domain.social.event.SkillUnsubscribedEvent;
+import org.springframework.context.ApplicationEventPublisher;
+import org.springframework.stereotype.Service;
+import org.springframework.transaction.annotation.Transactional;
+
+import java.util.List;
+
+@Service
+public class SkillSubscriptionService {
+    private final SkillSubscriptionRepository subscriptionRepository;
+    private final SkillRepository skillRepository;
+    private final ApplicationEventPublisher eventPublisher;
+
+    public SkillSubscriptionService(SkillSubscriptionRepository subscriptionRepository,
+                                    SkillRepository skillRepository,
+                                    ApplicationEventPublisher eventPublisher) {
+        this.subscriptionRepository = subscriptionRepository;
+        this.skillRepository = skillRepository;
+        this.eventPublisher = eventPublisher;
+    }
+
+    @Transactional
+    public void subscribe(Long skillId, String userId) {
+        ensureSkillExists(skillId);
+        if (subscriptionRepository.findBySkillIdAndUserId(skillId, userId).isPresent()) {
+            return; // idempotent
+        }
+        subscriptionRepository.save(new SkillSubscription(skillId, userId));
+        eventPublisher.publishEvent(new SkillSubscribedEvent(skillId, userId));
+    }
+
+    @Transactional
+    public void unsubscribe(Long skillId, String userId) {
+        ensureSkillExists(skillId);
+        subscriptionRepository.findBySkillIdAndUserId(skillId, userId).ifPresent(subscription -> {
+            subscriptionRepository.delete(subscription);
+            eventPublisher.publishEvent(new SkillUnsubscribedEvent(skillId, userId));
+        });
+    }
+
+    public boolean isSubscribed(Long skillId, String userId) {
+        ensureSkillExists(skillId);
+        return subscriptionRepository.findBySkillIdAndUserId(skillId, userId).isPresent();
+    }
+
+    public List<String> findSubscribersBySkillId(Long skillId) {
+        return subscriptionRepository.findAllBySkillId(skillId).stream()
+                .map(SkillSubscription::getUserId)
+                .distinct()
+                .toList();
+    }
+
+    private void ensureSkillExists(Long skillId) {
+        if (skillRepository.findById(skillId).isEmpty()) {
+            throw new DomainNotFoundException("skill.not_found", skillId);
+        }
+    }
+}