[Bug] [Bug] useTaskFeed serves stale MMKV-cached tasks indefinitely — cached data is never revalidated on mount if the store is non-empty #150

Why this matters now

The offline task feed cache is an explicit shipped feature. But the cache has no TTL and no revalidation trigger: if the store has tasks from a previous session, useTaskFeed serves them forever without ever calling the API again, even on a fresh app start with a working internet connection. New tasks added by the backend, status changes (open → closed), and reward amount updates are invisible until the user manually pulls to refresh.

Problem / What

src/hooks/useTaskFeed.ts:

useEffect(() => {
  if (tasksRef.current.length === 0) {
    void loadTasks(1);     // only fetches if the store is empty
  } else if (hasLocation) {
    lastFetchLocationRef.current = { lat: lat as number, lng: lng as number };
  }
  // eslint-disable-next-line react-hooks/exhaustive-deps
}, []);

If tasks.length > 0 (warm cache from a prior session), the mount effect skips the fetch entirely. There is no lastFetchedAt timestamp on taskStore and no revalidation logic.

The fix: add a tasksLastFetchedAt timestamp (ISO string) to taskStore via partializeTaskState (already exists) and set it in setTasks. In the mount effect, also trigger a background fetch when tasksLastFetchedAt is older than a configurable TASK_FEED_STALE_MS (e.g., 5 minutes). The background fetch should update the store silently — no loading skeleton during revalidation.

Key Challenges

• The background revalidation must not overwrite selectedTask if the user is mid-navigation to a task detail.
• TASK_FEED_STALE_MS should be a named constant, configurable in tests via a Jest module mock.
• The sanitizePersistedTaskState function already runs on rehydration — it must pass tasksLastFetchedAt through.
• useTaskFeed.test.ts and useTaskFeedRefetch.test.tsx must be extended to cover the stale-cache revalidation path.

Acceptance Criteria

• taskStore gains a tasksLastFetchedAt field, persisted to MMKV.
• useTaskFeed triggers a background fetch on mount when tasksLastFetchedAt is older than TASK_FEED_STALE_MS (5 min default), even if the store has tasks.
• The background fetch does not show a loading skeleton.
• selectedTask is not clobbered during background revalidation.
• Tests: (a) warm cache within TTL → no fetch, (b) warm cache past TTL → background fetch, (c) selectedTask preserved after revalidation.

Relevant files / functions

• src/hooks/useTaskFeed.ts — mount effect
• src/store/taskStore.ts — TaskState, partializeTaskState, sanitizePersistedTaskState
• src/__tests__/useTaskFeed.test.ts
• src/__tests__/useTaskFeedRefetch.test.tsx

Out of scope

• Server-sent events / real-time task updates.
• Per-filter TTL (all filters share the same tasksLastFetchedAt).