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/subsonic/scrobble_test.go

@@ -39,7 +39,7 @@ func testScrobblePool(t *testing.T) (*pgxpool.Pool, dbq.User, dbq.Track) {
 	dbtest.ResetDB(t, pool)
 	q := dbq.New(pool)
 	u, err := q.CreateUser(context.Background(), dbq.CreateUserParams{
-		Username: dbtest.TestUserPrefix + "tester", PasswordHash: "x", ApiToken: "x", IsAdmin: false,
+		Username: dbtest.TestUserPrefix + "tester", PasswordHash: "x", ApiTokenHash: "x", IsAdmin: false,
 	})
 	if err != nil {
 		t.Fatalf("user: %v", err)