fix: mirror #734 onto decomposed files (PG NaN slideshow seed, SQLite bool renders)

Same two pre-existing-on-main bugs, at their post-decomposition
locations: clampIntOrUndefined in adminEvents/crud.js slideshow seed;
!! coercion in EventDetailsHeader, EventInformationCard,
ClientAccessCard. Keeps this branch correct in either merge order with
#734 — when merging main afterwards, resolve the adminEvents.js
modify/delete conflict by keeping the deletion.

backend/src/utils/numericHelpers.js

@@ -31,4 +31,20 @@ function ensureNumber(value, fallback = 0) {
   return Number.isFinite(n) ? n : fallback;
 }
 
-module.exports = { ensureInt, ensureNumber };
+/**
+ * Parse a value as an integer clamped to [min, max]; `undefined` on
+ * anything that doesn't parse (null, undefined, '', booleans, garbage).
+ *
+ * Exists because the inline guard `Number.isFinite(+v) ? parseInt(v)`
+ * disagrees with itself for null/''/true (`+null` is 0 but
+ * `parseInt(null)` is NaN), which let NaN through Math.min/Math.max
+ * and into an INSERT — PostgreSQL rejects NaN for integer columns
+ * while SQLite silently stores NULL, so it only failed on PG.
+ */
+function clampIntOrUndefined(value, min, max) {
+  const n = parseInt(value, 10);
+  if (!Number.isFinite(n)) return undefined;
+  return Math.min(max, Math.max(min, n));
+}
+
+module.exports = { ensureInt, ensureNumber, clampIntOrUndefined };