fix(events): coerce expires_in_days to Number before addDays

The "Expires on" preview under the days-after-event input rendered
nonsense dates (e.g. 25.04.2026 + 120 days → 08.01.2095, ~68 years
out). Cause: handleInputChange stores e.target.value verbatim, which is
a string for <input type="number">, so formData.expires_in_days is "120"
not 120. date-fns addDays does:

  _date.setDate(_date.getDate() + amount)

When amount is a string, the + is string concatenation:
25 + "120" = "25120". setDate("25120") then sets day-of-month to 25120,
which carries over by ~68 years.

Fix: cast to Number at the call site. The validation/API-payload
codepaths already work because the comparisons at line 330 and the
JSON payload coerce numerically through different paths — only addDays
was actually broken.

The TypeScript type FormData.expires_in_days: number is a lie because
handleInputChange's [field]: e.target.value sets a string regardless.
Tightening that handler is a separate cleanup; this commit only fixes
the visible date bug.

frontend/src/pages/admin/CreateEventPage.tsx

@@ -780,7 +780,12 @@ export const CreateEventPage: React.FC = () => {
                 </div>
                 {formData.event_date && (
                   <p className="mt-2 text-sm text-neutral-500 dark:text-neutral-400">
-                    {t('events.expiresOn')}: {format(addDays(new Date(formData.event_date), formData.expires_in_days))}
+                    {/* Coerce to Number — handleInputChange stores the
+                        <input type="number"> value as a string, and date-fns
+                        addDays does `_date.setDate(_date.getDate() + amount)`
+                        which string-concatenates (25 + "120" = "25120") and
+                        ends up ~68 years in the future. */}
+                    {t('events.expiresOn')}: {format(addDays(new Date(formData.event_date), Number(formData.expires_in_days)))}
                   </p>
                 )}
               </div>