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/api/admin_users.go

@@ -169,7 +169,7 @@ func (h *handlers) handleAdminCreateUser(w http.ResponseWriter, r *http.Request)
 	user, err := q.CreateUserAdmin(r.Context(), dbq.CreateUserAdminParams{
 		Username:     req.Username,
 		PasswordHash: string(hash),
-		ApiToken:     apiToken,
+		ApiTokenHash: auth.HashAPIToken(apiToken),
 		IsAdmin:      req.IsAdmin,
 		DisplayName:  req.DisplayName,
 	})