Skip non-Tradovate firms until their trading logic exists

The master's catalog now labels each firm with a platform (TopstepX being
the first non-Tradovate one). Sync accepts the field, defaulting anything
missing or unrecognized to tradovate so an older master still validates,
and getTradableFirms only hands Tradovate firms to the client.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

lib/master-protocol.ts

@@ -78,6 +78,10 @@ export interface CatalogAccountType {
 export interface CatalogFirm {
     id: string;
     name: string;
+    /** Which platform the firm's accounts trade on. Only 'tradovate' firms get
+     *  a client here; anything else is synced but sits idle until its trading
+     *  logic exists. Older masters omit the field — treated as 'tradovate'. */
+    platform: 'tradovate' | 'topstepx';
     bannedSymbols: string[];
     accountTypes: CatalogAccountType[];
     updatedAt: string;