05/07 Phase 3: initial codes

app/security.py

@@ -104,3 +104,12 @@ def validate_setting_key(key: str) -> bool:
     """Return True if key is a valid setting key (alphanumeric, underscores, dots)."""
     import re
     return bool(key and re.match(r'^[a-zA-Z0-9_.]+$', key) and len(key) <= 100)
+
+
+def validate_passcode(passcode: str, min_len: int = 4, max_len: int = 6) -> bool:
+    """Return True if passcode is digits only and within the allowed length range."""
+    return (
+        bool(passcode)
+        and passcode.isdigit()
+        and min_len <= len(passcode) <= max_len
+    )