feat(m11): calendar + milestones with progress calc, tests, e2e

- CalendarRepository (event CRUD, range query, soft-delete, project isolation)
  + MilestoneRepository (CRUD, findToDosByMilestone) + ScheduleService
  (aggregated getCalendarEvents = custom events+milestones+todos in range,
  milestone CRUD, milestone_updated activity, progress auto-calc 0-100)
- APIs: calendar events GET/POST/PATCH/DELETE, milestones GET/POST/PATCH +
  progress
- Screens: calendar list view + CalendarEventForm, milestones list with
  progress bars + MilestoneForm, ProjectNav links
- Unit tests (CalendarRepository, MilestoneRepository, ScheduleService incl.
  progress calc 0/33/100) + e2e calendar

components/calendar/MilestoneForm.tsx

@@ -0,0 +1,73 @@
+'use client';
+
+import { useState, type FormEvent } from 'react';
+import { useRouter } from 'next/navigation';
+
+export function MilestoneForm({ projectId }: { projectId: number }) {
+  const router = useRouter();
+  const [title, setTitle] = useState('');
+  const [dueDate, setDueDate] = useState('');
+  const [loading, setLoading] = useState(false);
+  const [error, setError] = useState<string | null>(null);
+
+  async function onSubmit(event: FormEvent<HTMLFormElement>) {
+    event.preventDefault();
+    setLoading(true);
+    setError(null);
+    const res = await fetch(`/api/projects/${projectId}/milestones`, {
+      method: 'POST',
+      headers: { 'Content-Type': 'application/json' },
+      body: JSON.stringify({ title, dueDate: dueDate || null }),
+    });
+    setLoading(false);
+    if (res.ok) {
+      setTitle('');
+      setDueDate('');
+      router.refresh();
+    } else {
+      const b = (await res.json().catch(() => null)) as {
+        error?: { message?: string };
+      } | null;
+      setError(b?.error?.message ?? '作成に失敗しました');
+    }
+  }
+
+  return (
+    <form
+      onSubmit={onSubmit}
+      className="flex flex-col gap-2 rounded-lg border bg-white p-4 shadow-sm sm:flex-row sm:items-end"
+    >
+      <div className="flex-1">
+        <label className="block text-sm font-medium">マイルストーン名</label>
+        <input
+          type="text"
+          value={title}
+          onChange={(e) => setTitle(e.target.value)}
+          className="mt-1 w-full rounded border px-3 py-2"
+          required
+        />
+      </div>
+      <div>
+        <label className="block text-sm font-medium">期限</label>
+        <input
+          type="date"
+          value={dueDate}
+          onChange={(e) => setDueDate(e.target.value)}
+          className="mt-1 rounded border px-3 py-2"
+        />
+      </div>
+      <button
+        type="submit"
+        disabled={loading}
+        className="rounded bg-blue-600 px-4 py-2 font-medium text-white hover:bg-blue-700 disabled:opacity-50"
+      >
+        {loading ? '作成中...' : 'マイルストーン作成'}
+      </button>
+      {error && (
+        <p className="text-sm text-red-600" role="alert">
+          {error}
+        </p>
+      )}
+    </form>
+  );
+}