Equalize Access login failure handling

src/govoplan_access/backend/security/passwords.py

@@ -9,6 +9,14 @@ _ALGORITHM = "pbkdf2_sha256"
 _DEFAULT_ITERATIONS = 260_000
 _SALT_BYTES = 16
 
+# A valid, fixed-cost hash used when a login identity has no local password hash.
+# Its plaintext value is intentionally irrelevant; the hash only keeps failed
+# login attempts on the same verification path as existing local accounts.
+DUMMY_PASSWORD_HASH = (
+    "pbkdf2_sha256$260000$Z292b3BsYW4tZHVtbXktdjE="  # noqa: S105  # nosec B105 - non-account timing equalizer.
+    "$uWgE7ht8wO6cotOqNKK2yNomPt57gstVss5ben5gTbw="
+)
+
 
 def hash_password(password: str, *, iterations: int = _DEFAULT_ITERATIONS) -> str:
     salt = os.urandom(_SALT_BYTES)
@@ -37,4 +45,3 @@ def verify_password(password: str, encoded: str | None) -> bool:
         return False
     actual = hashlib.pbkdf2_hmac("sha256", password.encode("utf-8"), salt, iterations)
     return hmac.compare_digest(actual, expected)
-