Add Smart data checking mode

BACKLOG.md

@@ -113,12 +113,19 @@ Status: TODO / IN PROGRESS / DONE / BLOCKED.
   action in the Alerts tab that forces a fetch and immediately re-evaluates the
   cheapest-favourite alert. Copy should set expectation honestly: standard
   mode checks when FuelBoard refreshes prices (up to twice daily).
-- [ ] **Price-drop alerts: optional frequent-check mode** — if the app ever
-  relaxes the current "twice daily max" data policy, expose this as a clearly
-  separate mode rather than a freeform frequency slider. Example framing:
-  `Standard` (current behaviour) vs `Frequent` (best-effort extra checks),
-  with conservative caps (e.g. every 4–6 h, not hourly) and copy that does not
-  promise exact timing under iOS background scheduling.
+- [ ] **Data checking mode: Standard vs Smart** — keep the current cache-first
+  / 12 h policy as `Standard`, and add an optional `Smart` mode in Settings.
+  `Smart` should try lightweight background checks of the mirror pointer
+  (`latest.json` / `data_updated`) more often, and only download the full price
+  dataset when a newer snapshot is available. Design goal: better freshness
+  without pretending to do real-time refreshes or re-downloading the full dump
+  unnecessarily. User-facing copy must be explicit and honest:
+  - `Standard` — Saves battery. Refreshes price data up to twice daily.
+  - `Smart` — Checks for newer data more often in the background and refreshes
+    full prices only when an update is available.
+  Add a footnote/subcopy that background checks are best-effort and happen only
+  when iOS allows, so timing is not exact. Prefer this 2-mode setting over a
+  freeform frequency slider.
 - [ ] **Pull-to-refresh spinner state** — surface refresh in-flight state
   (currently `refreshable` fires but no visible progress in the row list).
 - [x] **Offline first-run** — bundled REAL 8,022-station dump (`FuelBoardDump`,