feat: epics, subtasks and checklists

Tickets can hang from another one of the same project, three levels deep
at most (epic → ticket → subtask). The ticket page lists its subtasks with
their progress, creates new ones and moves the ticket under an epic; a new
Épicas tab shows every epic and how far it has got. Each ticket also gets a
checklist for the small steps that don't deserve a ticket.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

libs/api/tracker/presenters/src/lib/graphql/inputs.ts

@@ -140,6 +140,13 @@ export class CreateIssueInput extends IssueFieldsInput {
   @Field(() => ID, { nullable: true }) @IsOptional() @IsUUID() releaseId?:
     | string
     | null;
+  @Field(() => ID, {
+    nullable: true,
+    description: 'Epic or ticket of the same project the new one breaks down',
+  })
+  @IsOptional()
+  @IsUUID()
+  parentId?: string | null;
 }
 
 @InputType()
@@ -173,6 +180,25 @@ export class ChangeIssueStatusInput {
   reason?: string | null;
 }
 
+@InputType()
+export class AddChecklistItemInput {
+  @Field(() => ID) @IsUUID() issueId!: string;
+  @Field() @IsString() @MinLength(1) @MaxLength(300) text!: string;
+}
+
+@InputType()
+export class UpdateChecklistItemInput {
+  @Field(() => ID) @IsUUID() issueId!: string;
+  @Field(() => ID) @IsUUID() itemId!: string;
+  @Field({ nullable: true })
+  @IsOptional()
+  @IsString()
+  @MinLength(1)
+  @MaxLength(300)
+  text?: string;
+  @Field({ nullable: true }) @IsOptional() @IsBoolean() done?: boolean;
+}
+
 @InputType()
 export class LinkIssuesInput {
   @Field(() => ID) @IsUUID() issueId!: string;
@@ -216,6 +242,13 @@ export class IssueFilterInput {
   @IsOptional()
   @IsString({ each: true })
   labels?: string[];
+  @Field(() => ID, {
+    nullable: true,
+    description: 'Only the tickets hanging from this one',
+  })
+  @IsOptional()
+  @IsUUID()
+  parentId?: string;
   @Field({ nullable: true, description: 'Matches key, title or description' })
   @IsOptional()
   @IsString()