perf(core): implement efficient bulk user deletion

Revamps the entire user deletion process to resolve critical performance bottlenecks that caused the web panel and database to freeze when removing multiple users.

- **Backend:** Core scripts (`kickuser.py`, `remove_user.py`) and the database layer are re-engineered to handle multiple users in a single, efficient batch operation using MongoDB's `delete_many`.
- **API:** A new `POST /api/v1/users/bulk-delete` endpoint is introduced for batch removals. The existing single-user `DELETE` endpoint is fixed to align with the new bulk logic.
- **Frontend:** The Users page now intelligently calls the bulk API when multiple users are selected, drastically improving UI responsiveness and reducing server load.

core/scripts/hysteria2/remove_user.py

@@ -5,27 +5,31 @@ import sys
 import os
 from db.database import db
 
-def remove_user(username):
+def remove_users(usernames):
     if db is None:
         return 1, "Error: Database connection failed. Please ensure MongoDB is running."
 
+    if not usernames:
+        return 1, "Error: No usernames provided for removal."
+
     try:
-        result = db.delete_user(username)
+        result = db.delete_users(usernames)
+        
         if result.deleted_count > 0:
-            return 0, f"User {username} removed successfully."
+            return 0, f"{result.deleted_count} user(s) removed successfully."
         else:
-            return 1, f"Error: User {username} not found."
+            return 1, "Error: No matching users found for removal."
 
     except Exception as e:
-        return 1, f"An error occurred while removing the user: {e}"
+        return 1, f"An error occurred while removing users: {e}"
 
 def main():
-    if len(sys.argv) != 2:
-        print(f"Usage: {sys.argv[0]} <username>")
+    if len(sys.argv) < 2:
+        print(f"Usage: {sys.argv[0]} <username1> [username2] ...")
         sys.exit(1)
 
-    username = sys.argv[1].lower()
-    exit_code, message = remove_user(username)
+    usernames = [username.lower() for username in sys.argv[1:]]
+    exit_code, message = remove_users(usernames)
     print(message)
     sys.exit(exit_code)