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/application/src/lib/facades/issue.facade.ts

@@ -6,6 +6,7 @@ import {
   IssueLink,
   IssuesGateway,
   IssueLinkType,
+  IssueSummary,
   UpdateIssueInput,
 } from '@task-stream/web/tracker/domain';
 import { map, Observable } from 'rxjs';
@@ -120,6 +121,20 @@ export class IssueFacade {
   }
 
   /** Places the open ticket under another one by its key; null: back to the top. */
+  /**
+   * Tickets to link to or hang the open one from: the most recently updated
+   * of all my projects (links may cross projects), the open one aside.
+   */
+  candidates(): Observable<IssueSummary[]> {
+    return this.issues
+      .mine({ assignedToMe: false, limit: 200 })
+      .pipe(
+        map((page) =>
+          page.items.filter((item) => item.id !== this.store.issue()?.id),
+        ),
+      );
+  }
+
   setParent(parentKey: string | null): void {
     this.withIssue((issueId) =>
       this.dispatch.parentRequested({ issueId, parentKey }),