feat: implement t-604 keyword prompt moderation

apps/moderation/normalization.py

@@ -0,0 +1,54 @@
+from __future__ import annotations
+
+import unicodedata
+from functools import lru_cache
+
+_ZERO_WIDTH_CHARS = {
+    "\u200b",
+    "\u200c",
+    "\u200d",
+    "\ufeff",
+    "\u2060",
+}
+
+
+@lru_cache(maxsize=1)
+def _opencc_converter():
+    try:
+        from opencc import OpenCC
+    except Exception:
+        return None
+    try:
+        return OpenCC("t2s")
+    except Exception:
+        return None
+
+
+def _to_simplified(value: str) -> str:
+    converter = _opencc_converter()
+    if converter is None:
+        return value
+    return converter.convert(value)
+
+
+def normalize_text(value: str | None) -> str:
+    """Normalize text before keyword matching.
+
+    The local keyword MVP removes obvious bypass characters only; it does not
+    attempt semantic rewriting or fuzzy matching.
+    """
+    if not value:
+        return ""
+
+    normalized = unicodedata.normalize("NFKC", value).lower()
+    normalized = _to_simplified(normalized)
+
+    chars: list[str] = []
+    for char in normalized:
+        if char in _ZERO_WIDTH_CHARS:
+            continue
+        category = unicodedata.category(char)
+        if category[0] in {"P", "Z"}:
+            continue
+        chars.append(char)
+    return "".join(chars)