refactor: enhance hackathon validation and data handling

- Updated hackathon validation logic to improve readability and maintainability.
- Changed date fields in HackathonHeader type from Date to string for better compatibility with API responses.
- Improved error handling in create and update hackathon functions to ensure proper validation and timezone handling.
- Refactored getFilteredHackathons function to streamline filtering and pagination logic.

Author: Riot-pls

server/services/date-parser.ts

@@ -0,0 +1,42 @@
+// All comments in English (as requested).
+export function getDateWithTimezone(dateStr: string, timeZone: string): Date {
+    const s: string = dateStr.trim();
+    const endsWithZ: boolean = /[zZ]$/.test(s);
+  
+    // Case 1: ends with 'Z' (UTC instant) → project to target TZ wall time
+    if (endsWithZ) {
+      const utcDate: Date = new Date(s);
+      if (isNaN(utcDate.getTime())) throw new Error(`Invalid date string: "${dateStr}"`);
+  
+      // Extract wall-clock parts in the target time zone
+      const fmt: Intl.DateTimeFormat = new Intl.DateTimeFormat('en-US', {
+        timeZone,
+        year: 'numeric',
+        month: '2-digit',
+        day: '2-digit',
+        hour: '2-digit',
+        minute: '2-digit',
+        second: '2-digit',
+        hour12: false,
+      });
+      const parts: Intl.DateTimeFormatPart[] = fmt.formatToParts(utcDate);
+      const map: Record<string, string> = Object.fromEntries(parts.map(p => [p.type, p.value]));
+  
+      const year: number = Number(map.year);
+      const month: number = Number(map.month);
+      const day: number = Number(map.day);
+      const hour: number = Number(map.hour ?? '0');
+      const minute: number = Number(map.minute ?? '0');
+      const second: number = Number(map.second ?? '0');
+      const ms: number = utcDate.getUTCMilliseconds();
+  
+      // Build a Date using those wall-clock numbers (this "freezes" the TZ wall time).
+      return new Date(year, month - 1, day, hour, minute, second, ms);
+    }
+  
+    // Case 2: any other format → ignore timeZone and parse as-is (preserve the instant)
+    const d: Date = new Date(s);
+    if (isNaN(d.getTime())) throw new Error(`Invalid date string: "${dateStr}"`);
+    return d;
+  }
+