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/identity/infrastructure/src/lib/mongo/user.schema.ts

@@ -2,8 +2,9 @@ import { Prop, Schema, SchemaFactory } from '@nestjs/mongoose';
 
 @Schema({ collection: 'users', timestamps: true, versionKey: false })
 export class UserDocument {
-  @Prop({ type: String, required: true })
-  _id!: string;
+  /** The user's id at Pireon; `_id` is Mongo's own ObjectId. */
+  @Prop({ type: String, required: true, unique: true })
+  id!: string;
 
   @Prop({ required: true })
   name!: string;