Fix event time save off-by-one using bundled tz data.

Use moment-timezone in parseEventDatetime so naive America/Asuncion wall-clock times convert correctly regardless of the host Node tz database. Also normalize user registration date filters with toDbDate for Postgres.

Co-authored-by: Cursor <cursoragent@cursor.com>

backend/src/lib/utils.ts

@@ -1,5 +1,6 @@
 import { nanoid } from 'nanoid';
 import { randomUUID } from 'crypto';
+import moment from 'moment-timezone';
 
 /**
  * Get database type (reads env var each time to handle module loading order)
@@ -59,17 +60,10 @@ export function parseEventDatetime(
     return new Date(datetime);
   }
 
-  // Treat the digits as UTC so we have a stable reference instant.
-  const fakeUTC = new Date(datetime + 'Z');
-
-  // Ask Intl what that UTC instant looks like in both UTC and the target tz.
-  const utcStr = fakeUTC.toLocaleString('en-US', { timeZone: 'UTC' });
-  const tzStr  = fakeUTC.toLocaleString('en-US', { timeZone: timezone });
-
-  // The gap between the two tells us the tz offset at this point in time.
-  const offsetMs = new Date(utcStr).getTime() - new Date(tzStr).getTime();
-
-  return new Date(fakeUTC.getTime() + offsetMs);
+  // Interpret the wall-clock digits as local time in `timezone` using
+  // moment-timezone's bundled IANA data. This keeps the conversion correct
+  // regardless of the host Node runtime's (possibly stale) tz database.
+  return moment.tz(datetime, timezone).toDate();
 }
 
 /**