fix(events): NaN from slideshow seed breaks event creation on PostgreSQL

The create route seeds show_interval_ms/show_transition_ms from
app_settings through an inline guard that pre-checked Number.isFinite(+v)
but then used parseInt(v). The two disagree for null/''/true — +null is 0
(finite) while parseInt(null) is NaN — so when the slideshow settings rows
are absent (getAppSetting returns its null default), NaN flowed through
Math.min/Math.max into the INSERT. PostgreSQL rejects NaN for integer
columns; SQLite silently stores NULL, which is why every SQLite-based
test passed while POST /api/admin/events 500'd on the PG dev stack and
broke the e2e smoke suite.

Fix: parse first, then check — clampIntOrUndefined in utils/numericHelpers
(unit-tested against every failure-mode input). Verified end-to-end: the
previously-failing minimal create now succeeds against the PG dev stack.

backend/__tests__/utils/numericHelpers.clampInt.test.js

@@ -0,0 +1,48 @@
+/**
+ * Regression tests for clampIntOrUndefined — the slideshow-seed NaN bug.
+ *
+ * The event-create route seeds show_interval_ms/show_transition_ms from
+ * app_settings via an int-parse-and-clamp. The old inline guard
+ * (`Number.isFinite(+v) ? parseInt(v) : undefined`) disagreed with itself
+ * for null/''/true: `+null` is 0 (finite) but `parseInt(null)` is NaN, so
+ * NaN flowed through Math.min/Math.max into the INSERT. PostgreSQL
+ * rejects NaN for integer columns ("invalid input syntax for type
+ * integer: NaN") while SQLite silently stores NULL — so POST
+ * /api/admin/events 500'd on PG whenever the slideshow settings rows
+ * were absent (getAppSetting returns its null default).
+ */
+
+const { clampIntOrUndefined } = require('../../src/utils/numericHelpers');
+
+describe('clampIntOrUndefined', () => {
+  it('returns undefined for null (the getAppSetting missing-row default)', () => {
+    expect(clampIntOrUndefined(null, 1000, 120000)).toBeUndefined();
+  });
+
+  it('returns undefined for undefined, empty string, and booleans', () => {
+    expect(clampIntOrUndefined(undefined, 1000, 120000)).toBeUndefined();
+    expect(clampIntOrUndefined('', 1000, 120000)).toBeUndefined();
+    expect(clampIntOrUndefined(true, 1000, 120000)).toBeUndefined();
+    expect(clampIntOrUndefined(false, 1000, 120000)).toBeUndefined();
+  });
+
+  it('returns undefined for non-numeric garbage', () => {
+    expect(clampIntOrUndefined('fast', 1000, 120000)).toBeUndefined();
+    expect(clampIntOrUndefined({}, 1000, 120000)).toBeUndefined();
+  });
+
+  it('never returns NaN for any of the failure-mode inputs', () => {
+    for (const v of [null, undefined, '', true, false, 'x', {}, []]) {
+      const out = clampIntOrUndefined(v, 100, 5000);
+      expect(Number.isNaN(out)).toBe(false);
+    }
+  });
+
+  it('parses and clamps valid values', () => {
+    expect(clampIntOrUndefined('2500', 1000, 120000)).toBe(2500);
+    expect(clampIntOrUndefined(2500, 1000, 120000)).toBe(2500);
+    expect(clampIntOrUndefined('500', 1000, 120000)).toBe(1000);
+    expect(clampIntOrUndefined(999999, 1000, 120000)).toBe(120000);
+    expect(clampIntOrUndefined('2500.9', 1000, 120000)).toBe(2500);
+  });
+});