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/user/http.go

@@ -22,6 +22,15 @@ func (h *HTTPHandler) RegisterRoutes(api fiber.Router) {
 	user.Get("/me", h.getAuthenticatedUser)
 }
 
+// getAuthenticatedUser returns the currently authenticated user
+// @Summary Get current user
+// @Description Retrieve the authenticated user's profile information
+// @Tags users
+// @Produce json
+// @Security BearerAuth
+// @Success 200 {object} User "User profile"
+// @Failure 401 {string} string "Not authenticated"
+// @Router /users/me [get]
 func (h *HTTPHandler) getAuthenticatedUser(c *fiber.Ctx) error {
 	u := reqctx.AuthenticatedUser(c).(*User)
 	if u == nil {