fix: corrige 6 bugs mineurs (BUG-035 a BUG-040)

backend/auth/password.py

@@ -1,14 +1,21 @@
 # backend/auth/password.py
 # Argon2id password hashing — OWASP 2024 recommended algorithm.
-# Parameters: time_cost=2, memory_cost=64MB, parallelism=2
+# Parameters (BUG-038): time_cost=2, memory_cost=19 MiB, parallelism=1
+# (OWASP current recommendation for Argon2id). The previous 64 MiB setting
+# allowed memory exhaustion under concurrent login attempts.
 
 from argon2 import PasswordHasher
 from argon2.exceptions import VerificationError, VerifyMismatchError
 
+#: Argon2id cost parameters (OWASP 2024: m=19456 KiB, t=2, p=1).
+ARGON2_TIME_COST = 2
+ARGON2_MEMORY_COST_KIB = 19456  # 19 MiB
+ARGON2_PARALLELISM = 1
+
 ph = PasswordHasher(
-    time_cost=2,
-    memory_cost=65536,  # 64 MB
-    parallelism=2,
+    time_cost=ARGON2_TIME_COST,
+    memory_cost=ARGON2_MEMORY_COST_KIB,
+    parallelism=ARGON2_PARALLELISM,
     hash_len=32,
     salt_len=16,
 )