feat(todo): richer metadata (tags, startDate, files) + edit/detail dialog

ToDoアイテムのメタデータを拡充し、編集/詳細ダイアログを追加。

- lib/db/migrations/003_todo_tags.sql: todo_items に tags 列を追加
- TodoRepository/TodoService: startDate(従来ハードコードNULL)とtagsをcreate/updateで永続化、fileIdsで添付紐付け(トランザクション)、deleteItemで添付クリーンアップ、getItem/getItemAttachments を追加
- API: GET /todos/items/[itemId] ({item, attachments}) を追加、POST/PATCH で startDate/tags/fileIds を受理、PATCH は null(クリア)と undefined(更新しない)を区別
- UI: TodoDialog(タイトル/説明/担当/優先度/開始日/期限/タグ/添付を編集、完了日時・タイムスタンプは読み取り専用)、KanbanBoard のカードクリックでダイアログを開きメタデータを表示
- MilestoneRepository の todo マッピングに tags を追加し as never キャストを修正
- docs/functional-design.md に tags / file_assets.source / attachments テーブルを追記

app/api/projects/[projectId]/todos/items/[itemId]/route.ts

@@ -6,6 +6,23 @@ import { handleApiError, jsonError } from '@/lib/api/handleError';
 
 export const runtime = 'nodejs';
 
+export async function GET(
+  _request: NextRequest,
+  { params }: { params: Promise<{ projectId: string; itemId: string }> }
+) {
+  const user = await getCurrentUser();
+  if (!user) return handleApiError(new UnauthorizedError());
+  const { itemId } = await params;
+  const service = createTodoService();
+  try {
+    const item = service.getItem(user.id, Number(itemId));
+    const attachments = service.getItemAttachments(user.id, Number(itemId));
+    return NextResponse.json({ item, attachments });
+  } catch (error) {
+    return handleApiError(error);
+  }
+}
+
 export async function PATCH(
   request: NextRequest,
   { params }: { params: Promise<{ projectId: string; itemId: string }> }
@@ -35,23 +52,29 @@ export async function PATCH(
       );
       return NextResponse.json({ item });
     }
+    // nullable フィールドは absent(undefined=更新しない) と null(クリア) を区別する
+    const nullableString = (v: unknown): string | null | undefined =>
+      v === undefined ? undefined : v === null ? null : String(v);
+    const nullableNumber = (v: unknown): number | null | undefined =>
+      v === undefined ? undefined : v === null ? null : Number(v);
+
     const item = service.updateItem(user.id, Number(itemId), {
       title: typeof body.title === 'string' ? body.title : undefined,
-      description:
-        typeof body.description === 'string' ? body.description : undefined,
-      assigneeId:
-        body.assigneeId === null || body.assigneeId === undefined
-          ? undefined
-          : Number(body.assigneeId),
+      description: nullableString(body.description),
+      assigneeId: nullableNumber(body.assigneeId),
       priority:
         typeof body.priority === 'string'
           ? (body.priority as 'low' | 'normal' | 'high')
           : undefined,
-      dueDate: typeof body.dueDate === 'string' ? body.dueDate : undefined,
-      milestoneId:
-        body.milestoneId === null || body.milestoneId === undefined
-          ? undefined
-          : Number(body.milestoneId),
+      startDate: nullableString(body.startDate),
+      dueDate: nullableString(body.dueDate),
+      tags: nullableString(body.tags),
+      milestoneId: nullableNumber(body.milestoneId),
+      fileIds: Array.isArray(body.fileIds)
+        ? body.fileIds
+            .map((n) => Number(n))
+            .filter((n) => Number.isFinite(n) && n > 0)
+        : undefined,
     });
     return NextResponse.json({ item });
   } catch (error) {