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>

apps/api/src/app/app.module.ts

@@ -47,6 +47,8 @@ const tracker = TrackerPresentersModule.withApplication(
         url: config.getOrThrow<string>('POSTGRES_URL'),
         autoLoadEntities: true,
         synchronize: config.get<boolean>('POSTGRES_SYNCHRONIZE'),
+        // uuid ids come from gen_random_uuid() (core since PostgreSQL 13).
+        uuidExtension: 'pgcrypto',
       }),
     }),