feat: uuid ids everywhere, pickers for linked tickets, @pi/ui checkboxes

- Postgres: every table has a uuid `id` (gen_random_uuid()); the event
  store keeps its relay order in `seq`, and the lookup tables their
  uniqueness in constraints of their own.
- MongoDB: `_id` is Mongo's ObjectId; `id` is the domain id (the same
  uuid as the aggregate's stream in Postgres), unique, and every query
  goes by it. Production starts from empty databases.
- Web: linking tickets and hanging one from a parent are picked from a
  searchable select (links across projects, parents within the project)
  instead of typing a key; every checkbox is @pi/ui's pi-checkbox.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

libs/api/tracker/infrastructure/src/lib/mongo/notifications.repository.ts

@@ -26,7 +26,7 @@ export class MongoNotificationsRepository extends NotificationsRepository {
 
   async insert({ id, ...rest }: NotificationReadModel): Promise<boolean> {
     const result = await this.notifications.updateOne(
-      { _id: id },
+      { id },
       { $setOnInsert: rest },
       { upsert: true },
     );
@@ -51,21 +51,21 @@ export class MongoNotificationsRepository extends NotificationsRepository {
 
   async markRead(userId: string, ids: string[] | null): Promise<void> {
     await this.notifications.updateMany(
-      { userId, readAt: null, ...(ids ? { _id: { $in: ids } } : {}) },
+      { userId, readAt: null, ...(ids ? { id: { $in: ids } } : {}) },
       { $set: { readAt: new Date().toISOString() } },
     );
   }
 
   async settings(userId: string): Promise<NotificationSettingsReadModel> {
     const doc = await this.settingsDocs
-      .findById(userId)
+      .findOne({ id: userId })
       .lean<NotificationSettingsDoc>();
     return { userId, email: doc?.email ?? false };
   }
 
   async saveSettings(settings: NotificationSettingsReadModel): Promise<void> {
     await this.settingsDocs.updateOne(
-      { _id: settings.userId },
+      { id: settings.userId },
       { $set: { email: settings.email } },
       { upsert: true },
     );