docs: add OpenAPI documentation with Scalar UI

- Add swaggo annotations to all HTTP handlers
- Add Swagger/OpenAPI spec generation with swag
- Create separate docs server binary (drexa-docs)
- Add Makefile with build, run, and docs targets
- Configure Scalar as the API documentation UI

Run 'make docs' to regenerate, 'make run-docs' to serve.

apps/backend/internal/upload/upload.go

@@ -2,17 +2,28 @@ package upload
 
 import "github.com/get-drexa/drexa/internal/virtualfs"
 
+// Status represents the upload state
+// @Description Upload status enumeration
 type Status string
 
 const (
-	StatusPending   Status = "pending"
+	// StatusPending indicates upload is awaiting content
+	StatusPending Status = "pending"
+	// StatusCompleted indicates upload finished successfully
 	StatusCompleted Status = "completed"
-	StatusFailed    Status = "failed"
+	// StatusFailed indicates upload failed
+	StatusFailed Status = "failed"
 )
 
+// Upload represents a file upload session
+// @Description File upload session with status and upload URL
 type Upload struct {
-	ID         string          `json:"id"`
-	Status     Status          `json:"status"`
-	TargetNode *virtualfs.Node `json:"-"`
-	UploadURL  string          `json:"uploadUrl"`
+	// Unique upload session identifier
+	ID string `json:"id" example:"xNq5RVBt3K88"`
+	// Current upload status
+	Status Status `json:"status" example:"pending" enums:"pending,completed,failed"`
+	// Internal target node reference
+	TargetNode *virtualfs.Node `json:"-" swaggerignore:"true"`
+	// URL to upload file content to
+	UploadURL string `json:"uploadUrl" example:"https://api.example.com/api/accounts/550e8400-e29b-41d4-a716-446655440000/uploads/xNq5RVBt3K88/content"`
 }