Fix AdminNotificationService crashing on string throttle_minutes config

env('ERROR_NOTIFICATION_THROTTLE_MINUTES', 15) returns the literal string
"15" from production's .env (env() doesn't cast numeric strings), which
Carbon 3.11's addMinutes() rejects with a TypeError. That crash happened
before the throttle-dedup cache write, so it silently masked the *original*
exception on every notify-worthy error, indefinitely (discovered while
investigating a missing workshop-registration email — the real SMTP error
was hidden behind this crash). Cast to (int).

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

docs/tasks/statuses.md

@@ -134,3 +134,4 @@
 | 20 | 20 Nablus Workshop Form - Match Site Background and Card Styling | Done |
 | 21 | 21 Nablus Workshop Form - Flyer Size Adjustment | Done |
 | 22 | 22 Fix Footer Address - Add Missing Street Name | Done |
+| 23 | 23 Fix AdminNotificationService Carbon TypeError Masking Exceptions | Done |