Add models package with embedded DB interface pattern

- internal/models/model.go: DB interface + Base struct for all models
- internal/models/channel.go: Channel model with DB access for relation queries
- Database.NewChannel() factory injects db reference into model instances
- Uses interface to avoid circular imports (models -> db)

internal/models/channel.go

@@ -0,0 +1,21 @@
+package models
+
+import (
+	"time"
+)
+
+// Channel represents a chat channel.
+type Channel struct {
+	Base
+	ID        int64     `json:"id"`
+	Name      string    `json:"name"`
+	Topic     string    `json:"topic"`
+	Modes     string    `json:"modes"`
+	CreatedAt time.Time `json:"created_at"`
+	UpdatedAt time.Time `json:"updated_at"`
+}
+
+// Example relation method — will be fleshed out when we add channel_members:
+// func (c *Channel) Members(ctx context.Context) ([]*User, error) {
+//     return c.DB().GetChannelMembers(ctx, c.ID)
+// }