feat: add peppered PBKDF2 password hashing

Implements password hashing with PBKDF2-SHA256 and pepper-based additional security.
Includes password verification, rehash detection, and pepper loading from environment
or file.

Claude-Session: https://claude.ai/code/session_01M9MLit5Ko3X4s7rzC5Kv7X

backend/tests/test_passwords.py

@@ -0,0 +1,58 @@
+import os
+import stat
+import tempfile
+import unittest
+from pathlib import Path
+from unittest import mock
+
+from auth import ITERATIONS, hash_password, load_pepper, needs_rehash, verify_password
+
+PEPPER = b"p" * 32
+# Independently computed: HMAC-SHA256(PEPPER, password) -> PBKDF2-SHA256, 1000 iterations, 16 zero-byte salt.
+KNOWN_ANSWER = "pbkdf2_sha256$1000$AAAAAAAAAAAAAAAAAAAAAA==$9ZLLEusnEPU3Km8h+vnd4ue9fw7rHdm4QwuBX5ozynE="
+
+
+class PasswordHashTests(unittest.TestCase):
+    def test_known_answer(self):
+        stored = hash_password("correct horse battery staple", PEPPER, salt=bytes(16), iterations=1000)
+        self.assertEqual(stored, KNOWN_ANSWER)
+
+    def test_round_trip_uses_current_iterations_and_random_salt(self):
+        first = hash_password("correct horse battery staple", PEPPER)
+        second = hash_password("correct horse battery staple", PEPPER)
+        self.assertTrue(first.startswith(f"pbkdf2_sha256${ITERATIONS}$"))
+        self.assertNotEqual(first, second)
+        self.assertTrue(verify_password("correct horse battery staple", first, PEPPER))
+
+    def test_wrong_password_fails(self):
+        self.assertFalse(verify_password("wrong password!!", KNOWN_ANSWER, PEPPER))
+
+    def test_wrong_pepper_fails(self):
+        self.assertFalse(verify_password("correct horse battery staple", KNOWN_ANSWER, b"q" * 32))
+
+    def test_needs_rehash_below_current_iterations(self):
+        self.assertTrue(needs_rehash(KNOWN_ANSWER))
+        self.assertFalse(needs_rehash(hash_password("x" * 12, PEPPER)))
+
+
+class PepperTests(unittest.TestCase):
+    def test_env_pepper_must_be_long_enough(self):
+        with mock.patch.dict(os.environ, {"PASSWORD_PEPPER": "short"}):
+            with self.assertRaisesRegex(RuntimeError, "at least 32 characters"):
+                load_pepper(Path("/nonexistent"))
+
+    def test_env_pepper_is_used_when_set(self):
+        with mock.patch.dict(os.environ, {"PASSWORD_PEPPER": "e" * 40}):
+            self.assertEqual(load_pepper(Path("/nonexistent")), b"e" * 40)
+
+    def test_file_fallback_creates_private_stable_pepper(self):
+        with tempfile.TemporaryDirectory() as tmp, mock.patch.dict(os.environ):
+            os.environ.pop("PASSWORD_PEPPER", None)
+            pepper_file = Path(tmp) / ".pepper"
+            with self.assertLogs("auth", level="WARNING"):
+                first = load_pepper(pepper_file)
+            with self.assertLogs("auth", level="WARNING"):
+                second = load_pepper(pepper_file)
+            self.assertEqual(first, second)
+            self.assertEqual(len(first), 64)
+            self.assertEqual(stat.S_IMODE(pepper_file.stat().st_mode), 0o600)