fix(subscriptions): address code review feedback

- Fix platform-specific %%-d format → use .day and .year directly in templates and messages
- Fix Tailwind JIT dynamic class interpolation → use static class variables in showFlash()
- Fix Jinja pending_date rendering → use .strftime('%B') + .day + .year
- Add aria-atomic=true to flash container for full screen-reader announcements
- Move SessionLocal() creation inside try block in Celery task for proper session management

Co-authored-by: christianlouis <361235+christianlouis@users.noreply.github.com>

app/utils/subscription.py

@@ -742,7 +742,7 @@ def request_subscription_change(
         "new_tier": new_tier_id,
         "message": (
             f"Your downgrade to {get_tier(new_tier_id, db)['name']} has been scheduled for "
-            f"{change_date.strftime('%B %-d, %Y')}. "
+            f"{change_date.strftime('%B')} {change_date.day}, {change_date.year}. "
             "You will continue to have access to your current plan until then."
         ),
     }