fix(auth): hashlib.scrypt 在 LibreSSL 构建下缺失导致注册返回 500

macOS 自带 Python 3.9 链接 LibreSSL，hashlib 不提供 scrypt，
register 调用 hash_password 时抛 AttributeError，被 Flask 兜成
500 internal error（登录随之全部失败）。

改用 PBKDF2-HMAC-SHA256（每个 CPython 构建都有），并采用自描述的
存储格式 pbkdf2_sha256$iterations$salt$hash，便于日后迁移算法。
verify_password 仍兼容旧的 salt:hash scrypt 格式（当解释器支持时）。

Co-Authored-By: Claude Haiku 4.5 <noreply@anthropic.com>

backend/auth.py

@@ -1,9 +1,17 @@
 """
-Authentication helpers: scrypt password hashing, JWT signing/verification,
+Authentication helpers: password hashing, JWT signing/verification,
 and the require_auth decorator used by route blueprints.
 
-Password hashing mirrors the original Node implementation exactly:
-    salt (16 random bytes, hex) : scrypt(password, salt, n=16384, r=8, p=1, dklen=64, hex)
+Hashes are stored in a self-describing format so the algorithm can be
+migrated later without invalidating existing rows:
+
+    pbkdf2_sha256$<iterations>$<salt_hex>$<hash_hex>
+
+PBKDF2-HMAC-SHA256 is used because it is available in every CPython build.
+`hashlib.scrypt` is NOT: builds linked against LibreSSL (notably the system
+Python on macOS) omit it, which made registration fail with a 500.
+Legacy `<salt_hex>:<hash_hex>` scrypt hashes are still verified when the
+running interpreter supports scrypt.
 """
 import hashlib
 import hmac
@@ -16,6 +24,11 @@ from functools import wraps
 
 from config import JWT_SECRET, JWT_EXPIRY_DAYS
 
+PBKDF2_ITERATIONS = 200_000
+PBKDF2_PREFIX = "pbkdf2_sha256"
+_SALT_BYTES = 16
+_DK_LEN = 64
+
 
 class AuthError(Exception):
     def __init__(self, code, message):
@@ -24,21 +37,21 @@ class AuthError(Exception):
         self.message = message
 
 
+def _pbkdf2(password: str, salt: bytes, iterations: int) -> str:
+    return hashlib.pbkdf2_hmac(
+        "sha256", password.encode("utf-8"), salt, iterations, dklen=_DK_LEN
+    ).hex()
+
+
 def hash_password(password: str) -> str:
-    salt = os.urandom(16)
-    derived = hashlib.scrypt(
-        password.encode("utf-8"),
-        salt=salt,
-        n=16384,
-        r=8,
-        p=1,
-        dklen=64,
-    )
-    return f"{salt.hex()}:{derived.hex()}"
+    salt = os.urandom(_SALT_BYTES)
+    derived = _pbkdf2(password, salt, PBKDF2_ITERATIONS)
+    return f"{PBKDF2_PREFIX}${PBKDF2_ITERATIONS}${salt.hex()}${derived}"
 
 
-def verify_password(password: str, stored: str) -> bool:
-    if not stored or ":" not in stored:
+def _verify_legacy_scrypt(password: str, stored: str) -> bool:
+    """Verify a pre-migration `salt_hex:hash_hex` scrypt hash, if supported."""
+    if not hasattr(hashlib, "scrypt"):
         return False
     salt_hex, hash_hex = stored.split(":", 1)
     try:
@@ -46,14 +59,28 @@ def verify_password(password: str, stored: str) -> bool:
     except ValueError:
         return False
     derived = hashlib.scrypt(
-        password.encode("utf-8"),
-        salt=salt,
-        n=16384,
-        r=8,
-        p=1,
-        dklen=64,
-    )
-    return hmac.compare_digest(derived.hex(), hash_hex)
+        password.encode("utf-8"), salt=salt, n=16384, r=8, p=1, dklen=_DK_LEN
+    ).hex()
+    return hmac.compare_digest(derived, hash_hex)
+
+
+def verify_password(password: str, stored: str) -> bool:
+    if not stored:
+        return False
+
+    if stored.startswith(PBKDF2_PREFIX + "$"):
+        try:
+            _, iterations, salt_hex, hash_hex = stored.split("$", 3)
+            salt = bytes.fromhex(salt_hex)
+            derived = _pbkdf2(password, salt, int(iterations))
+        except (ValueError, TypeError):
+            return False
+        return hmac.compare_digest(derived, hash_hex)
+
+    if ":" in stored:
+        return _verify_legacy_scrypt(password, stored)
+
+    return False
 
 
 def sign_token(user_id: str) -> str: