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/tracker/infrastructure/src/lib/mongo/find.repositories.ts

@@ -79,7 +79,7 @@ export class MongoFindProjectsRepository extends FindProjectsRepository {
   }
 
   async findById(id: string): Promise<ProjectReadModel | null> {
-    const doc = await this.projects.findById(id).lean<ProjectDoc>();
+    const doc = await this.projects.findOne({ id: id }).lean<ProjectDoc>();
     return doc ? toProject(doc) : null;
   }
 
@@ -109,7 +109,7 @@ export class MongoFindProjectsRepository extends FindProjectsRepository {
     userId: string,
   ): Promise<ProjectRole | null> {
     const doc = await this.projects
-      .findOne({ _id: projectId, memberIds: userId }, { members: 1 })
+      .findOne({ id: projectId, memberIds: userId }, { members: 1 })
       .lean<Pick<ProjectDoc, 'members'>>();
     return doc?.members.find((m) => m.user.id === userId)?.role ?? null;
   }
@@ -124,7 +124,7 @@ export class MongoFindIssuesRepository extends FindIssuesRepository {
   }
 
   async findById(id: string): Promise<IssueReadModel | null> {
-    const doc = await this.issues.findById(id).lean<IssueDoc>();
+    const doc = await this.issues.findOne({ id: id }).lean<IssueDoc>();
     return doc ? issueFromDoc(doc) : null;
   }
 
@@ -136,7 +136,7 @@ export class MongoFindIssuesRepository extends FindIssuesRepository {
   async findByIds(ids: string[]): Promise<IssueReadModel[]> {
     if (ids.length === 0) return [];
     const docs = await this.issues
-      .find({ _id: { $in: ids } })
+      .find({ id: { $in: ids } })
       .lean<IssueDoc[]>();
     return docs.map(issueFromDoc);
   }
@@ -322,15 +322,15 @@ export class MongoFindIssuesRepository extends FindIssuesRepository {
     // Only the top of each tree: a ticket with subtasks under an epic is not one.
     const docs = await this.issues
       .find({
-        _id: { $in: [...progress.keys()] },
+        id: { $in: [...progress.keys()] },
         'parent.id': null,
         trashedAt: null,
       })
       .sort({ number: 1 })
       .lean<IssueDoc[]>();
     return docs.map((doc) => {
-      const { _id: _ignored, ...row } = progress.get(doc._id) ?? {
-        _id: doc._id,
+      const { _id: _ignored, ...row } = progress.get(doc.id) ?? {
+        _id: doc.id,
         total: 0,
         finished: 0,
         storyPoints: 0,
@@ -456,7 +456,7 @@ export class MongoFindCommentsRepository extends FindCommentsRepository {
   }
 
   async findById(id: string): Promise<CommentReadModel | null> {
-    const doc = await this.comments.findById(id).lean<CommentDoc>();
+    const doc = await this.comments.findOne({ id: id }).lean<CommentDoc>();
     return doc ? commentFromDoc(doc) : null;
   }
 
@@ -496,7 +496,7 @@ export class MongoFindReleasesRepository extends FindReleasesRepository {
   }
 
   async findById(id: string): Promise<ReleaseReadModel | null> {
-    const doc = await this.releases.findById(id).lean<ReleaseDoc>();
+    const doc = await this.releases.findOne({ id: id }).lean<ReleaseDoc>();
     return doc ? fromDoc(doc) : null;
   }