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/projects.page.ts

@@ -17,6 +17,7 @@ import {
 } from '@angular/forms/signals';
 import {
   PiActionButtonComponent,
+  PiCheckboxComponent,
   PiEmptyStateComponent,
   PiLabelDirective,
   PiSkeletonComponent,
@@ -34,6 +35,7 @@ import { MyWorkComponent } from './my-work';
   selector: 'ts-projects-page',
   changeDetection: ChangeDetectionStrategy.OnPush,
   imports: [
+    PiCheckboxComponent,
     FormField,
     GoToComponent,
     ProjectCardComponent,
@@ -123,14 +125,13 @@ import { MyWorkComponent } from './my-work';
         >
           <pi-label>Descripción</pi-label>
         </pi-textarea>
-        <label class="ts-public-check">
-          <input type="checkbox" [formField]="f.isPublic" />
-          <span>
-            <strong>Proyecto público</strong>
+        <div class="ts-public-check">
+          <pi-checkbox label="Proyecto público" [formField]="f.isPublic" />
+          <p>
             Cualquiera, también sin iniciar sesión, podrá verlo (solo lectura).
             Se puede cambiar después en Configuración.
-          </span>
-        </label>
+          </p>
+        </div>
         <pi-action-button
           size="w100"
           color="primary"
@@ -164,22 +165,11 @@ import { MyWorkComponent } from './my-work';
       }
     }
     .ts-public-check {
-      display: flex;
-      align-items: flex-start;
-      gap: 0.625rem;
-      font-size: 0.8125rem;
-      color: var(--ts-muted);
-      cursor: pointer;
-      input {
-        width: 1rem;
-        height: 1rem;
-        margin-top: 0.125rem;
-        accent-color: var(--ts-brand);
-      }
-      strong {
-        display: block;
-        color: var(--ts-ink);
-        font-size: 0.875rem;
+      p {
+        /* Under the checkbox's text, past the box. */
+        margin: 0 0 0 2.5rem;
+        color: var(--ts-muted);
+        font-size: 0.8125rem;
       }
     }
     .ts-projects {