chore(auth): drop dead UpdateProfileRequest; rename shared struct

The combined profile handler (self-edit PUT /api/auth/profile and
admin PUT /api/auth/profile/:id) binds one struct for both paths —
and it's the admin-named one. Its twin UpdateProfileRequest had zero
references left. Delete it and rename AdminUpdateUserRequest to
ProfileUpdateRequest so the name matches who actually binds it; the
comments describing field semantics now stand on their own instead of
pointing at the deleted twin. Wire format (json/form tags) unchanged;
docs schema registration follows the rename.

internal/docs/schema.go

@@ -705,7 +705,7 @@ var Schemas = map[string]Schema{
 	},
 	"PUT /api/auth/profile": {
 		// Shared with admin variant (PUT /auth/profile/:id).
-		Request:  handlers.AdminUpdateUserRequest{},
+		Request:  handlers.ProfileUpdateRequest{},
 		Response: json.RawMessage(`{"message": ""}`),
 	},
 	"DELETE /api/auth/profile": {

internal/handlers/auth.go

@@ -124,19 +124,7 @@ type UserProfile struct {
 	DefaultAudioSpeed float64 `json:"default_audio_speed,omitempty"`
 }
 
-type UpdateProfileRequest struct {
-	Username  string `json:"username,omitempty" form:"username" validate:"omitempty,min=3,max=50"`
-	Email     string `json:"email,omitempty" form:"email" validate:"omitempty,email"`
-	FirstName string `json:"first_name,omitempty" form:"first_name" validate:"omitempty,max=100"`
-	LastName  string `json:"last_name,omitempty" form:"last_name" validate:"omitempty,max=100"`
-	Theme     string `json:"theme,omitempty" form:"theme" validate:"omitempty"`
-	Timezone  string `json:"timezone,omitempty" form:"timezone" validate:"omitempty"`
-	// Per the SPEEDS list bounds; the profile select always sends it,
-	// JSON clients omit it (0 = untouched).
-	DefaultAudioSpeed float64 `json:"default_audio_speed,omitempty" form:"default_audio_speed" validate:"omitempty,min=0.25,max=4"`
-}
-
-type AdminUpdateUserRequest struct {
+type ProfileUpdateRequest struct {
 	Username  string `json:"username,omitempty" form:"username" validate:"omitempty,min=3,max=50"`
 	Email     string `json:"email,omitempty" form:"email" validate:"omitempty,email"`
 	FirstName string `json:"first_name,omitempty" form:"first_name" validate:"omitempty,max=100"`
@@ -144,7 +132,8 @@ type AdminUpdateUserRequest struct {
 	Theme     string `json:"theme,omitempty" form:"theme" validate:"omitempty"`
 	Timezone  string `json:"timezone,omitempty" form:"timezone" validate:"omitempty"`
 	Role      string `json:"role,omitempty" form:"role" validate:"omitempty,oneof=user admin"`
-	// Same semantics as UpdateProfileRequest.DefaultAudioSpeed.
+	// Per the SPEEDS list bounds; the profile select always sends it,
+	// JSON clients omit it (0 = untouched).
 	DefaultAudioSpeed float64 `json:"default_audio_speed,omitempty" form:"default_audio_speed" validate:"omitempty,min=0.25,max=4"`
 }
 
@@ -581,7 +570,7 @@ func (h *AuthHandler) UpdateProfile(c *echo.Context) error {
 		targetUserUUID = currentUser.ID
 	}
 
-	var req AdminUpdateUserRequest
+	var req ProfileUpdateRequest
 	if err := c.Bind(&req); err != nil {
 		return c.JSON(http.StatusBadRequest, map[string]string{"error": "invalid request"})
 	}