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/mongo-user.registry.ts

@@ -16,20 +16,20 @@ export class MongoUserRegistry extends UserRegistry {
 
   async upsert(user: User): Promise<void> {
     await this.users.updateOne(
-      { _id: user.id },
+      { id: user.id },
       { $set: { name: user.name, email: user.email, isActive: user.isActive } },
       { upsert: true },
     );
   }
 
   async findById(id: string): Promise<User | null> {
-    const doc = await this.users.findById(id).lean();
+    const doc = await this.users.findOne({ id }).lean();
     return doc ? this.toUser(doc) : null;
   }
 
   async findByIds(ids: string[]): Promise<User[]> {
     if (ids.length === 0) return [];
-    const docs = await this.users.find({ _id: { $in: ids } }).lean();
+    const docs = await this.users.find({ id: { $in: ids } }).lean();
     return docs.map((d) => this.toUser(d));
   }
 
@@ -48,7 +48,7 @@ export class MongoUserRegistry extends UserRegistry {
 
   private toUser(doc: UserDocument): User {
     return this.factory.restore({
-      id: doc._id,
+      id: doc.id,
       name: doc.name,
       email: doc.email,
       isActive: doc.isActive,