fix(security): bump cryptography 46.0.7 -> 50.0.0 to clear 4 CVEs

The pip-audit step added in E1 S4 flagged cryptography 46.0.7 for PYSEC-2026-3552
/3553/3554 and GHSA-537c-gmf6-5ccf; all are fixed by 50.0.0. The requirements.txt
ceiling `<47` was blocking the upgrade — relaxed to `>=50.0.0,<51`. The lock is a
surgical bump (only the cryptography line changed; cffi 2.1.1 already satisfies
it, no other package moved). pip-audit -r requirements.lock is now clean.

Add tests/test_security.py (there was none for the crypto primitives): the
Fernet encrypt/decrypt round-trip, decrypt_optional degrading on garbage and on a
key mismatch (the B-core-2 path), and the argon2 password hash/verify round-trip
— so a future crypto-lib bump can't silently break the token-at-rest path. Gate
green: 330 pytest.

backend/requirements.lock

@@ -20,7 +20,7 @@ certifi==2026.7.22
 cffi==2.1.1
 charset-normalizer==3.5.1
 click==8.4.2
-cryptography==46.0.7
+cryptography==50.0.0
 fastapi==0.141.1
 feedparser-sgmllib==2.1.0
 feedparser==6.0.14

backend/requirements.txt

@@ -11,7 +11,7 @@ feedparser>=6.0,<7.0
 apscheduler>=3.10,<4.0
 py3langid>=0.3,<1.0
 itsdangerous>=2.1,<3.0
-cryptography>=46.0.7,<47
+cryptography>=50.0.0,<51
 argon2-cffi>=23.1,<26
 # Download center: yt-dlp does the extraction/download; ffmpeg (OS-level in the Dockerfile)
 # does the merge + postprocessing. The [default] extra pulls yt-dlp-ejs (the challenge-solver

backend/tests/test_security.py

@@ -0,0 +1,36 @@
+"""Crypto primitives — a guard for the cryptography (Fernet) + argon2 dependency bumps.
+
+Added with the cryptography 46->50 CVE bump: encrypt/decrypt/decrypt_optional had NO test, and they are
+the token-at-rest path (Google/API/SMTP/Plex secrets in system_config, OAuth refresh tokens). A fresh
+Fernet key is monkeypatched in so the test doesn't depend on TOKEN_ENCRYPTION_KEY being set in the env.
+Pure-logic (no DB).
+"""
+from cryptography.fernet import Fernet
+
+from app import security
+
+
+def test_fernet_encrypt_decrypt_roundtrip(monkeypatch):
+    monkeypatch.setattr(security, "_fernet", Fernet(Fernet.generate_key()))
+    assert security.decrypt(security.encrypt("hunter2")) == "hunter2"
+    assert security.encrypt(None) is None
+    assert security.decrypt(None) is None
+
+
+def test_decrypt_optional_degrades_instead_of_raising(monkeypatch):
+    # Garbage / absent input -> None (never raises).
+    monkeypatch.setattr(security, "_fernet", Fernet(Fernet.generate_key()))
+    assert security.decrypt_optional("not-a-valid-token") is None
+    assert security.decrypt_optional(None) is None
+    # And a ciphertext written under a DIFFERENT key (the TOKEN_ENCRYPTION_KEY-mismatch case that B-core-2
+    # made non-fatal) also degrades to None rather than raising InvalidToken.
+    ciphertext = security.encrypt("secret")
+    monkeypatch.setattr(security, "_fernet", Fernet(Fernet.generate_key()))  # rotate the key
+    assert security.decrypt_optional(ciphertext) is None
+
+
+def test_password_hash_and_verify_roundtrip():
+    hashed = security.hash_password("correct horse battery")
+    assert security.verify_password("correct horse battery", hashed) is True
+    assert security.verify_password("wrong password", hashed) is False
+    assert security.verify_password("anything", None) is False