feat: add channel support

backend/src/main/java/com/openisle/controller/ChannelController.java

@@ -0,0 +1,35 @@
+package com.openisle.controller;
+
+import com.openisle.dto.ChannelDto;
+import com.openisle.model.User;
+import com.openisle.repository.UserRepository;
+import com.openisle.service.ChannelService;
+import lombok.RequiredArgsConstructor;
+import org.springframework.security.core.Authentication;
+import org.springframework.web.bind.annotation.*;
+
+import java.util.List;
+
+@RestController
+@RequestMapping("/api/channels")
+@RequiredArgsConstructor
+public class ChannelController {
+    private final ChannelService channelService;
+    private final UserRepository userRepository;
+
+    private Long getCurrentUserId(Authentication auth) {
+        User user = userRepository.findByUsername(auth.getName())
+                .orElseThrow(() -> new IllegalArgumentException("User not found"));
+        return user.getId();
+    }
+
+    @GetMapping
+    public List<ChannelDto> listChannels(Authentication auth) {
+        return channelService.listChannels(getCurrentUserId(auth));
+    }
+
+    @PostMapping("/{channelId}/join")
+    public ChannelDto joinChannel(@PathVariable Long channelId, Authentication auth) {
+        return channelService.joinChannel(channelId, getCurrentUserId(auth));
+    }
+}