fix(ai): align provider default model fallbacks

The BYOK Anthropic client fell back to the retired claude-3-5-sonnet-latest;
match the chain-runner default (claude-sonnet-4-6). The admin ai-settings GET
displayed a gpt-4o-mini fallback that disagreed with the actual provider
default (gpt-4o); align the display string so the surfaced model matches what
the provider would use.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/app/api/admin/ai-settings/route.ts

@@ -18,7 +18,7 @@ export const GET = apiHandler(async () => {
   });
 
   const hasKey = Boolean(settings?.adminAiKeyEncrypted);
-  const model = settings?.adminAiModel ?? "gpt-4o-mini";
+  const model = settings?.adminAiModel ?? "gpt-4o";
   const baseUrl = settings?.adminAiBaseUrl ?? "https://api.openai.com/v1";
 
   return apiSuccess({

src/lib/ai/provider.ts

@@ -52,7 +52,7 @@ function buildUserProvider(row: UserAIRow): AIProvider | null {
       // the SDK default is correct.
       return new AnthropicClient({
         apiKey: decrypt(row.aiAnthropicKeyEncrypted),
-        model: row.aiModel ?? "claude-3-5-sonnet-latest",
+        model: row.aiModel ?? "claude-sonnet-4-6",
       });
     }
     case "LOCAL": {