feat: add unread message count API

- Added `GET /messages/unread-count` endpoint to retrieve the count of unread messages for a user.
- Implemented `getUnreadCount` method in `MessagesService` and `MessagesRepository`.
- Updated frontend service to support fetching unread message count via API.

backend/src/messages/repositories/messages.repository.ts

@@ -133,4 +133,35 @@ export class MessagesRepository {
 			.from(conversationParticipants)
 			.where(eq(conversationParticipants.conversationId, conversationId));
 	}
+
+	async markAsRead(conversationId: string, userId: string) {
+		await this.databaseService.db
+			.update(messages)
+			.set({ readAt: new Date() })
+			.where(
+				and(
+					eq(messages.conversationId, conversationId),
+					sql`${messages.senderId} != ${userId}`,
+					sql`${messages.readAt} IS NULL`,
+				),
+			);
+	}
+
+	async countUnreadMessages(userId: string) {
+		const result = await this.databaseService.db
+			.select({ count: sql<number>`count(*)` })
+			.from(messages)
+			.innerJoin(
+				conversationParticipants,
+				eq(messages.conversationId, conversationParticipants.conversationId),
+			)
+			.where(
+				and(
+					eq(conversationParticipants.userId, userId),
+					sql`${messages.senderId} != ${userId}`,
+					sql`${messages.readAt} IS NULL`,
+				),
+			);
+		return Number(result[0]?.count || 0);
+	}
 }