Fix QA review warnings: model server_default, calendar_id coercion

- EventTemplate model: add server_default=func.now() to created_at
  to match migration 011 and prevent autogenerate drift
- CalendarPage: use nullish coalescing for template calendar_id
  instead of || 0 which produced an invalid falsy ID

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

backend/app/models/event_template.py

@@ -1,4 +1,4 @@
-from sqlalchemy import String, Text, Boolean, ForeignKey, Integer
+from sqlalchemy import String, Text, Boolean, ForeignKey, Integer, func
 from sqlalchemy.orm import Mapped, mapped_column
 from datetime import datetime
 from typing import Optional
@@ -21,4 +21,4 @@ class EventTemplate(Base):
         Integer, ForeignKey("locations.id", ondelete="SET NULL"), nullable=True
     )
     is_starred: Mapped[bool] = mapped_column(Boolean, default=False)
-    created_at: Mapped[datetime] = mapped_column(default=datetime.now)
+    created_at: Mapped[datetime] = mapped_column(default=datetime.now, server_default=func.now())

frontend/src/components/calendar/CalendarPage.tsx

@@ -297,7 +297,7 @@ export default function CalendarPage() {
       title: template.title,
       description: template.description || '',
       all_day: template.all_day,
-      calendar_id: template.calendar_id || 0,
+      calendar_id: template.calendar_id ?? undefined,
       location_id: template.location_id || undefined,
       is_starred: template.is_starred,
       recurrence_rule: template.recurrence_rule || undefined,