perf: server-side caching + remove bulk status fetch from experiment page

- Add backend/src/lib/cache.js — in-process TTL Map cache with
  prefix-based invalidation; no external dependencies

- Cache /experiments/:id/calendar?field=X per (experimentId, field).
  Invalidated immediately on any DailyStatus write.

- Cache /experiments/:id/daily-statuses per (experimentId, date, analysisOnly).
  Invalidated immediately on any DailyStatus write (create/update/delete
  and analysis-summary saves all clear the relevant prefix).

- Add ?analysisOnly=true to /daily-statuses: filters rows where
  analysis_summary IS NOT NULL. Charts are the only consumer and already
  discard rows without analysis data; pre-filtering server-side avoids
  sending the full status payload (71 rows → 1 for this experiment).

- ExperimentCalendar: remove allStatuses prop, fetch calendar data
  directly via API. The component now issues one small GET per field
  selection change instead of the parent loading thousands of rows upfront.

- ExperimentDetail: pass analysisOnly:true to getDailyStatuses;
  drop allStatuses prop from ExperimentCalendar. Cold experiment page
  load no longer fetches every daily status record.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/src/lib/cache.js

@@ -0,0 +1,33 @@
+// Lightweight in-process TTL cache with prefix-based invalidation.
+// Single-process only — suitable for this single-container deployment.
+
+const DEFAULT_TTL_MS = 10 * 60 * 1000; // 10 minutes
+
+const store = new Map(); // key → { value, expiresAt }
+
+function set(key, value, ttlMs = DEFAULT_TTL_MS) {
+  store.set(key, { value, expiresAt: Date.now() + ttlMs });
+}
+
+function get(key) {
+  const entry = store.get(key);
+  if (!entry) return undefined;
+  if (Date.now() > entry.expiresAt) {
+    store.delete(key);
+    return undefined;
+  }
+  return entry.value;
+}
+
+function del(key) {
+  store.delete(key);
+}
+
+// Remove all keys that start with prefix — used to invalidate a whole experiment's worth of cached data.
+function delByPrefix(prefix) {
+  for (const key of store.keys()) {
+    if (key.startsWith(prefix)) store.delete(key);
+  }
+}
+
+module.exports = { set, get, del, delByPrefix };