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/infrastructure/db/repositories/user_repository.py

@@ -10,7 +10,7 @@ import uuid
 from sqlalchemy import func, select
 from sqlalchemy.ext.asyncio import AsyncSession
 
-from app.domain.entities import Credentials, User
+from app.domain.entities import Credentials, SubsonicCredentials, User
 from app.domain.errors import NotFoundError
 from app.infrastructure.db.models import UserModel
 
@@ -91,3 +91,22 @@ class SqlAlchemyUserRepository:
         return (
             await self._session.execute(select(func.count()).select_from(UserModel))
         ).scalar_one()
+
+    async def get_subsonic_credentials_by_username(
+        self, username: str
+    ) -> SubsonicCredentials | None:
+        row = (
+            await self._session.execute(select(UserModel).where(UserModel.username == username))
+        ).scalar_one_or_none()
+        if row is None:
+            return None
+        return SubsonicCredentials(user=_to_entity(row), password_enc=row.subsonic_password_enc)
+
+    async def get_subsonic_password_enc(self, user_id: uuid.UUID) -> str | None:
+        row = await self._get_row(user_id)
+        return row.subsonic_password_enc
+
+    async def set_subsonic_password_enc(self, user_id: uuid.UUID, password_enc: str) -> None:
+        row = await self._get_row(user_id)
+        row.subsonic_password_enc = password_enc
+        await self._session.flush()