feat(m12): meetings + schedule conflict detection, tests, e2e

- MeetingRepository (CRUD, soft-delete, members, findMeetingsByUserInRange
  overlap) + CalendarRepository.findByCreatorInRange + TodoRepository.
  findHighPriorityByAssignee
- MeetingService (createMeeting tx+members, meeting_invited notify,
  meeting_created activity, SSE meeting.created; updateMeeting/updateMinutes/
  deleteMeeting; checkScheduleConflicts = other meetings overlap + calendar
  events overlap + high-priority todos due +-3d, time-overlap logic,
  excludeMeetingId; conflicts are warnings)
- APIs: meetings list/create/edit + /check conflict check
- Screens: meetings page + MeetingForm(member checkboxes, conflict display)
  + ConflictWarning, ProjectNav link
- Unit tests (MeetingRepository, MeetingService incl. conflict algorithm)
  + e2e meetings

components/meetings/ConflictWarning.tsx

@@ -0,0 +1,46 @@
+export interface ScheduleConflict {
+  userId: number;
+  type: 'meeting' | 'calendar_event' | 'important_todo';
+  refId: number;
+  title: string;
+  startAt: string;
+  endAt: string | null;
+}
+
+const TYPE_LABELS: Record<string, string> = {
+  meeting: 'ミーティング',
+  calendar_event: 'カレンダーイベント',
+  important_todo: '期限の近い重要タスク',
+};
+
+export function ConflictWarning({
+  conflicts,
+}: {
+  conflicts: ScheduleConflict[];
+}) {
+  if (conflicts.length === 0) {
+    return (
+      <p className="text-sm text-green-600" data-testid="no-conflicts">
+        スケジュールの重複はありません。
+      </p>
+    );
+  }
+  return (
+    <div
+      className="rounded border border-yellow-300 bg-yellow-50 p-3"
+      data-testid="conflict-warning"
+    >
+      <p className="text-sm font-semibold text-yellow-800">
+        スケジュールの重複があります（作成は完了しています）:
+      </p>
+      <ul className="mt-1 list-inside list-disc text-sm text-yellow-800">
+        {conflicts.map((c, i) => (
+          <li key={i}>
+            [{TYPE_LABELS[c.type] ?? c.type}] {c.title}（{c.startAt}
+            {c.endAt ? ` 〜 ${c.endAt}` : ''}）
+          </li>
+        ))}
+      </ul>
+    </div>
+  );
+}