Refactor: Modularize lib_general.py by extracting core functionalities.

- Extracted Email functions to app/lib_email.py.
- Extracted Export functions to app/lib_export.py.
- Extracted JWT utilities to app/lib_jwt.py.
- Extracted Hash utilities to app/lib_hash.py.
- Updated app/lib_general.py to import from these new modules for backward compatibility.
- Updated V3 Frontend API Guide with latest security and site_domain exception details.

app/lib_hash.py

@@ -0,0 +1,16 @@
+from passlib.hash import argon2
+import logging
+
+log = logging.getLogger(__name__)
+
+# Moved from lib_general.py 2026-01-07
+def secure_hash_string(string: str) -> str:
+    string_hash = argon2.using(rounds=14, memory_cost=1536, parallelism=2).hash(string)
+    return string_hash
+
+# Moved from lib_general.py 2026-01-07
+def verify_secure_hash_string(string: str, string_hash: str) -> bool:
+    if argon2.verify(string, string_hash):
+        return True
+    else:
+        return False