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/web/tracker/presenters/src/lib/pages/backlog.page.ts

@@ -10,6 +10,7 @@ import {
 } from '@angular/core';
 import { RouterLink } from '@angular/router';
 import {
+  PiCheckboxComponent,
   PiEmptyStateComponent,
   PiIconButtonComponent,
   PiIconComponent,
@@ -34,6 +35,7 @@ import { initials, PRIORITY, statusLook, typeLook } from '../ui/copy';
   selector: 'ts-backlog-page',
   changeDetection: ChangeDetectionStrategy.OnPush,
   imports: [
+    PiCheckboxComponent,
     BulkBarComponent,
     RouterLink,
     CdkDrag,
@@ -96,14 +98,15 @@ import { initials, PRIORITY, statusLook, typeLook } from '../ui/copy';
             [cdkDragData]="issue"
             [class.is-selected]="selected().has(issue.id)"
           >
-            <input
-              type="checkbox"
+            <pi-checkbox
               class="ts-backlog__pick"
-              [attr.aria-label]="'Seleccionar ' + issue.key"
               [disabled]="!project.canContribute()"
               [checked]="selected().has(issue.id)"
-              (change)="toggle(issue.id, $any($event.target).checked)"
-            />
+              (checkedChange)="toggle(issue.id, $event)"
+              ><span class="visually-hidden"
+                >Seleccionar {{ issue.key }}</span
+              ></pi-checkbox
+            >
             <span class="ts-backlog__pos ts-mono">{{ i + 1 }}</span>
             <span
               class="ts-backlog__type"