Fix compare place names (Nominatim burst) + weekly duplicate location name (#29)

Compare was the only page loading several cells at once, so its concurrent
reverse-geocode calls burst past Nominatim's ~1 req/sec limit, got rate-limited,
and cached a null label — leaving those locations stuck on bare coordinates.

- climate.py: serialize + rate-limit reverse_geocode behind a lock (>=1.1s between
  Nominatim calls, re-checking the cache under the lock), so concurrent callers
  resolve reliably instead of bursting.
- store.py: shorten the reverse-geocode miss TTL 1 day -> 1 hour so a transient
  null retries soon; add a `cache` flag to put_payload.
- app.py: don't persist a calendar payload whose place failed to resolve, so the
  coordinates fallback can't stick for the life of the token.

Weekly showed the place name twice (top label beside the Find button AND the
results <h2>). Keep the <h2> (it carries the coords + climatology context) as the
single name display; the top label now only holds the transient coordinates
written on selection and is hidden once the named results render.


Claude-Session: https://claude.ai/code/session_01B3Q2EkHHnTUX2BfhV5q1Zc

Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>

static/app.js

@@ -121,9 +121,12 @@ function placeLabel(data) {
 function render(data) {
   recentData = data;
   forecastData = null;   // invalidate any forecast from a previous location/date
-  // Reflect the chosen place beside the Find button (which becomes "Change").
-  locLabel.textContent = placeLabel(data);
-  locLabel.hidden = false;
+  // The place name appears once — as the results heading (<h2>) below, which also
+  // carries the coordinates + climatology context. The top label only holds the
+  // transient coordinates written on selection, so hide it now that the named
+  // results have rendered (otherwise the name would show twice).
+  locLabel.hidden = true;
+  locLabel.textContent = "";
   findBtn.innerHTML = `${window.LocationPicker.PIN_ICON} <span>Change location</span>`;
   const c = data.climatology;
   const cell = data.cell;