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/settings-schema.ts

@@ -289,6 +289,14 @@ export const DEFAULT_PROJECT_SETTINGS = {
       requestTimeoutMs: 30000,
     },
   },
+  evalSettings: {
+    enabled: false,
+    intervalMs: 86_400_000,
+    evaluatorProvider: undefined,
+    evaluatorModelId: undefined,
+    followUpPolicy: "suggest-only",
+    retentionDays: 30,
+  },
   researchEnabled: true,
   researchMaxConcurrentRuns: 3,
   researchDefaultTimeout: 300000,