feat(preview): log spec validation failures with raw output

422s from /preview hid the actual reason: zod_message tells which field
was wrong and a 400-char preview of the model output reveals refusals
or non-JSON returns. Both stay in the api log only — never surfaced
to the client unchanged.

apps/api/src/routes/servers.ts

@@ -121,9 +121,30 @@ export async function serverRoutes(app: FastifyInstance): Promise<void> {
       });
     } catch (err) {
       if (err instanceof SpecValidationError) {
+        // Log the actual Zod validation failure so we can see *which* field
+        // the model got wrong. Without this we can only see "422" in the
+        // logs and can't tell whether to fix the prompt, the schema, or
+        // the model output cleanup. Prompt is truncated to keep PII risk
+        // bounded.
+        app.log.warn(
+          {
+            zod_message: err.message,
+            prompt: parsed.data.prompt.slice(0, 200),
+            model: choice.displayName,
+          },
+          'preview_spec_invalid',
+        );
         return reply.code(422).send({ error: 'spec_invalid', detail: err.message });
       }
       if (err instanceof BannedPatternError) {
+        app.log.warn(
+          {
+            reason: err.message,
+            prompt: parsed.data.prompt.slice(0, 200),
+            model: choice.displayName,
+          },
+          'preview_banned_pattern',
+        );
         return reply.code(422).send({ error: 'banned_pattern', detail: err.message });
       }
       if (err instanceof SpecTimeoutError) {