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/api/schemas/subsonic.py

@@ -0,0 +1,13 @@
+"""Schemas for Subsonic app-password self-service (native /api/v1 surface).
+
+The Subsonic /rest layer itself returns its own XML/JSON envelope, not these
+pydantic models — these only back the lifecycle endpoints that reveal/rotate the
+recoverable app-password."""
+
+from pydantic import BaseModel
+
+
+class SubsonicPasswordResponse(BaseModel):
+    """The plaintext Subsonic app-password, for pasting into a client."""
+
+    password: str