feat(handlers): implement consolidated user profile endpoints

Implement DeleteUser, ResetUserPassword, and UpdateUserAdmin handlers.
Update collections handler to check soft-deleted users. Update dashboard
service to exclude deleted users from statistics.

internal/handlers/collections.go

@@ -12,6 +12,7 @@ import (
 	"time"
 
 	"github.com/google/uuid"
+	"github.com/jackc/pgx/v5"
 	"github.com/jackc/pgx/v5/pgtype"
 	"github.com/labstack/echo/v4"
 )
@@ -848,6 +849,14 @@ func (h *CollectionHandler) PreviewCollection(c echo.Context) error {
 		return c.JSON(http.StatusBadRequest, map[string]string{"error": "invalid library_id"})
 	}
 
+	_, err = h.db.GetLibrary(c.Request().Context(), pgtype.UUID{Bytes: libUUID, Valid: true})
+	if err != nil {
+		if err == pgx.ErrNoRows {
+			return c.JSON(http.StatusBadRequest, map[string]string{"error": "library not found"})
+		}
+		return c.JSON(http.StatusInternalServerError, map[string]string{"error": err.Error()})
+	}
+
 	if req.Limit <= 0 || req.Limit > 100 {
 		req.Limit = 20
 	}