feat(channel): integrate channel functionality into post management

- Added Channel support in Post model, including ChannelID field in PostRequest and PostResponse DTOs.
- Updated PostService and PostRepository to handle channel-specific logic for creating and listing posts.
- Introduced ChannelRepository and ChannelService, along with corresponding handlers for managing channels.
- Enhanced router to include routes for channel management and public channel listing.
- Seeded default channels in the database during initialization.

internal/model/channel.go

@@ -0,0 +1,32 @@
+package model
+
+import (
+	"time"
+
+	"github.com/google/uuid"
+	"gorm.io/gorm"
+)
+
+// Channel 频道配置（用于帖子分区）
+type Channel struct {
+	ID          string    `json:"id" gorm:"type:varchar(36);primaryKey"`
+	Name        string    `json:"name" gorm:"type:varchar(100);not null"`
+	Slug        string    `json:"slug" gorm:"type:varchar(100);uniqueIndex;not null"`
+	Description string    `json:"description" gorm:"type:varchar(255)"`
+	SortOrder   int       `json:"sort_order" gorm:"default:0;index"`
+	IsActive    bool      `json:"is_active" gorm:"default:true;index"`
+	CreatedAt   time.Time `json:"created_at" gorm:"autoCreateTime"`
+	UpdatedAt   time.Time `json:"updated_at" gorm:"autoUpdateTime"`
+}
+
+func (c *Channel) BeforeCreate(tx *gorm.DB) error {
+	if c.ID == "" {
+		c.ID = uuid.New().String()
+	}
+	return nil
+}
+
+func (Channel) TableName() string {
+	return "channels"
+}
+