feat(auth): store Subsonic API keys hashed; a new key is shown once (M462 #4983)

users.api_token held each user's apiKey in plaintext and was looked up by
equality, so a leaked row or backup handed out working keys. Migration
0063 replaces it with api_token_hash (sha256, hex), computed in place
from the existing keys so every Subsonic client keeps working.

The key can no longer be read back: GET /api/me/api-token is gone, and
POST returns the new key once. Settings shows it right after Regenerate
with a copy button and a "won't be shown again" note.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

internal/playsessions/service_test.go

@@ -42,7 +42,7 @@ func seedTestUser(t *testing.T, pool *pgxpool.Pool) pgtype.UUID {
 	u, err := dbq.New(pool).CreateUser(context.Background(), dbq.CreateUserParams{
 		Username:     dbtest.TestUserPrefix + "tester",
 		PasswordHash: "x",
-		ApiToken:     "x",
+		ApiTokenHash: "x",
 		IsAdmin:      false,
 	})
 	if err != nil {