feat: allow user-configurable AI model selection with server-side validation

apps/server/src/routers/index.ts

@@ -1,13 +1,21 @@
 import { protectedProcedure, publicProcedure, router } from "../lib/trpc";
+import { ALLOWED_MODELS, DEFAULT_MODEL } from "../lib/ai-models";
 
 export const appRouter = router({
   healthCheck: publicProcedure.query(() => {
     return "OK";
   }),
+  allowedModels: publicProcedure.query(() => {
+    return {
+      models: ALLOWED_MODELS,
+      defaultModel: DEFAULT_MODEL,
+    } as const;
+  }),
   privateData: protectedProcedure.query(({ ctx }) => {
     return {
       message: "This is private",
       user: ctx.user,
+      aiModel: ctx.aiModel ?? null,
     };
   }),
 });