Add login lockout and encrypt Player.totp_secret/auth_token at rest

Per-username lockout (5 failed TOTP attempts / 5 min) stops account-targeted
brute force regardless of source IP. Player.totp_secret is now Fernet-
encrypted (ENCRYPTION_KEY env, db/crypto.py) instead of stored in plaintext,
and auth_token is stored as a SHA-256 hash rather than the raw session token.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

db/crypto.py

@@ -0,0 +1,42 @@
+"""Sifrovanie/hashovanie citlivych stlpcov (Player.totp_secret, Player.auth_token).
+
+totp_secret sa musi dat spatne desifrovat (treba ho na vygenerovanie/overenie
+TOTP kodu), preto Fernet -- symetricke sifrovanie s klucom z env ENCRYPTION_KEY.
+auth_token sa iba porovnava, nikdy nepotrebujeme povodnu hodnotu spat, preto
+staci jednosmerny SHA-256 hash (token ma 384 bitov entropie z
+secrets.token_urlsafe(48) v api/auth.py, takze netreba salt/pepper).
+"""
+
+import hashlib
+import os
+
+from cryptography.fernet import Fernet, InvalidToken
+
+_ENV_VAR = "ENCRYPTION_KEY"
+
+
+def _fernet() -> Fernet:
+    key = os.environ.get(_ENV_VAR)
+    if not key:
+        raise RuntimeError(
+            f"{_ENV_VAR} nie je nastaveny. Vygeneruj ho pomocou:\n"
+            '  python -c "from cryptography.fernet import Fernet; print(Fernet.generate_key().decode())"\n'
+            "a nastav ako env premennu (drz ho stabilny -- zmena znamena, "
+            "ze existujuce totp_secret sa uz nedaju desifrovat)."
+        )
+    return Fernet(key.encode())
+
+
+def encrypt(plaintext: str) -> str:
+    return _fernet().encrypt(plaintext.encode()).decode()
+
+
+def decrypt(ciphertext: str) -> str:
+    try:
+        return _fernet().decrypt(ciphertext.encode()).decode()
+    except InvalidToken as exc:
+        raise ValueError("Neplatny alebo poskodeny sifrovany udaj.") from exc
+
+
+def hash_token(token: str) -> str:
+    return hashlib.sha256(token.encode()).hexdigest()