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>

tests/fakes.py

@@ -4,13 +4,14 @@ import datetime as dt
 import uuid
 from dataclasses import dataclass, replace
 
-from app.domain.entities import Credentials, User
+from app.domain.entities import Credentials, SubsonicCredentials, User
 
 
 @dataclass
 class _Stored:
     user: User
     password_hash: str
+    subsonic_password_enc: str | None = None
 
 
 class InMemoryUserRepository:
@@ -61,6 +62,22 @@ class InMemoryUserRepository:
     async def count(self) -> int:
         return len(self._by_id)
 
+    async def get_subsonic_credentials_by_username(
+        self, username: str
+    ) -> SubsonicCredentials | None:
+        for stored in self._by_id.values():
+            if stored.user.username == username:
+                return SubsonicCredentials(
+                    user=stored.user, password_enc=stored.subsonic_password_enc
+                )
+        return None
+
+    async def get_subsonic_password_enc(self, user_id: uuid.UUID) -> str | None:
+        return self._by_id[user_id].subsonic_password_enc
+
+    async def set_subsonic_password_enc(self, user_id: uuid.UUID, password_enc: str) -> None:
+        self._by_id[user_id].subsonic_password_enc = password_enc
+
 
 @dataclass
 class _Token: