feat(subsonic): per-user encrypted app-password foundation

Subsonic auth (t=md5(password+salt), legacy p=) needs a recoverable secret,
but login passwords are stored as a one-way argon2 hash. Add a separate,
per-user app-password: high-entropy, random, and encrypted at rest with a
Fernet key derived from SUBSONIC_SECRET_KEY (never stored in the DB).

- SubsonicPasswordCipher + generate_subsonic_password in core.security
- users.subsonic_password_enc column (+ Alembic migration), repo + port methods
- SubsonicAuthService: verify (t+s / p / p=enc:) and rotate/reveal lifecycle
- self-service GET/POST /users/me/subsonic-password + admin rotate endpoint
- domain SubsonicCredentials + SubsonicCipher port; deps wiring

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

app/core/security.py

@@ -6,16 +6,54 @@ to this module (CLAUDE.md: security is a cross-cutting concern in ``core``).
 Higher layers depend only on the Protocols, never on pwdlib/pyjwt directly.
 """
 
+import base64
 import datetime as dt
+import hashlib
+import secrets
 import uuid
 
 import jwt
+from cryptography.fernet import Fernet, InvalidToken
 from pwdlib import PasswordHash
 
 from app.core.config import Settings
 from app.domain.errors import AuthenticationError
 from app.domain.tokens import IssuedToken, TokenClaims, TokenType
 
+# Length (in bytes of entropy) of a generated Subsonic app-password. 18 bytes of
+# url-safe base64 → 24 characters, well above the Subsonic auth threat model.
+_SUBSONIC_PASSWORD_ENTROPY_BYTES = 18
+
+
+def generate_subsonic_password() -> str:
+    """A fresh, high-entropy Subsonic app-password (url-safe, ~24 chars)."""
+    return secrets.token_urlsafe(_SUBSONIC_PASSWORD_ENTROPY_BYTES)
+
+
+class SubsonicPasswordCipher:
+    """Symmetric encrypt/decrypt for the recoverable Subsonic app-password.
+
+    Subsonic auth (``t=md5(password+salt)`` and legacy ``p=``) needs the plaintext
+    password server-side, so — unlike the argon2-hashed login password — the
+    app-password is stored *encrypted*, not hashed. A Fernet key (AES-128-CBC +
+    HMAC) is derived from the configured secret; the plaintext key never touches
+    the DB. Implements :class:`app.domain.ports.SubsonicCipher`.
+    """
+
+    def __init__(self, secret_key: str) -> None:
+        digest = hashlib.sha256(secret_key.encode("utf-8")).digest()
+        self._fernet = Fernet(base64.urlsafe_b64encode(digest))
+
+    def encrypt(self, plaintext: str) -> str:
+        return self._fernet.encrypt(plaintext.encode("utf-8")).decode("ascii")
+
+    def decrypt(self, token: str) -> str:
+        try:
+            return self._fernet.decrypt(token.encode("ascii")).decode("utf-8")
+        except InvalidToken as exc:
+            # Wrong/rotated secret key, or corrupted ciphertext.
+            raise AuthenticationError("Stored Subsonic password could not be decrypted.") from exc
+
 
 class Argon2PasswordHasher:
     """argon2id hasher with sensible defaults from pwdlib."""