feat(FN-3393): add eval settings with scheduled eval project settings UI

Adds a scheduled eval settings feature (FN-3393) with a new `evalSettings` contract in core, type-safe numeric validation, a project settings UI section, and an API route for persisting the payload. The bulk of the changes are in the settings modal and supporting test coverage. Two smaller, unrelate

Fusion-Task-Id: FN-3393

packages/core/src/eval-settings.ts

@@ -0,0 +1,23 @@
+import { resolveValidatorSettingsModel } from "./model-resolution.js";
+import type { ResolvedEvalSettings, Settings } from "./types.js";
+
+const DEFAULT_EVAL_SETTINGS: Omit<ResolvedEvalSettings, "evaluatorProvider" | "evaluatorModelId"> = {
+  enabled: false,
+  intervalMs: 86_400_000,
+  followUpPolicy: "suggest-only",
+  retentionDays: 30,
+};
+
+export function resolveEvalSettings(settings: Partial<Settings> | undefined): ResolvedEvalSettings {
+  const scopedSettings = settings?.evalSettings;
+  const validatorModel = resolveValidatorSettingsModel(settings);
+
+  return {
+    enabled: scopedSettings?.enabled ?? DEFAULT_EVAL_SETTINGS.enabled,
+    intervalMs: scopedSettings?.intervalMs ?? DEFAULT_EVAL_SETTINGS.intervalMs,
+    evaluatorProvider: scopedSettings?.evaluatorProvider ?? validatorModel.provider,
+    evaluatorModelId: scopedSettings?.evaluatorModelId ?? validatorModel.modelId,
+    followUpPolicy: scopedSettings?.followUpPolicy ?? DEFAULT_EVAL_SETTINGS.followUpPolicy,
+    retentionDays: scopedSettings?.retentionDays ?? DEFAULT_EVAL_SETTINGS.retentionDays,
+  };
+}