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>

apps/api/schema.gql

@@ -13,6 +13,11 @@ type Activity {
   type: String!
 }
 
+input AddChecklistItemInput {
+  issueId: ID!
+  text: String!
+}
+
 type AreaOption {
   archived: Boolean!
   id: String!
@@ -57,6 +62,23 @@ input ChangeIssueStatusInput {
   status: String!
 }
 
+"""A step inside a ticket that does not deserve a ticket of its own"""
+type ChecklistItem {
+  done: Boolean!
+  id: ID!
+  text: String!
+}
+
+"""How far the tickets hanging from one have got"""
+type ChildProgress {
+  """Done or cancelled"""
+  finished: Int!
+  finishedStoryPoints: Int!
+  percent: Int!
+  storyPoints: Int!
+  total: Int!
+}
+
 """
 Writes are acknowledged immediately; the read model catches up asynchronously. Wait for a subscription event with version >= this version to see the change.
 """
@@ -121,6 +143,9 @@ input CreateIssueInput {
   """Figma frame for UI changes"""
   figmaUrl: String
   labels: [String!]
+
+  """Epic or ticket of the same project the new one breaks down"""
+  parentId: ID
   priority: IssuePriority
   projectId: ID!
   releaseId: ID
@@ -172,6 +197,12 @@ type DurationStats {
   p85Days: Float
 }
 
+"""A top-level ticket that groups others, with their progress"""
+type Epic {
+  issue: Issue!
+  progress: ChildProgress!
+}
+
 type FlowSeries {
   counts: [Int!]!
   status: String!
@@ -185,6 +216,7 @@ type Issue {
   area: String
   assignee: UserRef
   attachments: [Attachment!]!
+  checklist: [ChecklistItem!]!
   commentCount: Int!
   comments(limit: Int! = 50, offset: Int! = 0): CommentPage!
   createdAt: String!
@@ -204,6 +236,9 @@ type Issue {
   linkedIssues: [Issue!]!
   links: [IssueLink!]!
   number: Int!
+
+  """The epic or ticket this one breaks down"""
+  parent: IssueParent
   priority: IssuePriority!
   projectId: ID!
   projectKey: String!
@@ -237,6 +272,9 @@ input IssueFilterInput {
   assigneeId: ID
   categories: [StatusCategory!]
   labels: [String!]
+
+  """Only the tickets hanging from this one"""
+  parentId: ID
   priorities: [IssuePriority!]
   projectId: ID!
   releaseId: ID
@@ -269,6 +307,13 @@ type IssuePage {
   total: Int!
 }
 
+"""The ticket another one hangs from"""
+type IssueParent {
+  id: ID!
+  key: String!
+  title: String!
+}
+
 enum IssuePriority {
   CRITICAL
   HIGH
@@ -317,6 +362,8 @@ type Me {
 }
 
 type Mutation {
+  """Add a step to a ticket's checklist"""
+  addChecklistItem(input: AddChecklistItemInput!): CommandResult!
   addComment(body: String!, issueId: ID!): CommandResult!
 
   """Add a Pireon user to the project"""
@@ -357,14 +404,23 @@ type Mutation {
   Remove a file from a ticket (whoever uploaded it, or an owner/admin). Upload with the REST API.
   """
   removeAttachment(attachmentId: ID!, issueId: ID!): CommandResult!
+  removeChecklistItem(issueId: ID!, itemId: ID!): CommandResult!
   removeProjectMember(projectId: ID!, userId: ID!): CommandResult!
 
+  """
+  Place a ticket under an epic or another ticket of the same project (null: back to the top). Three levels at most: epic → ticket → subtask.
+  """
+  setIssueParent(issueId: ID!, parentId: ID): CommandResult!
+
   """Put a ticket in a release (null removes it)"""
   setIssueRelease(issueId: ID!, releaseId: ID): CommandResult!
   unlinkIssues(input: LinkIssuesInput!): CommandResult!
 
   """Stop following a ticket"""
   unwatchIssue(issueId: ID!): CommandResult!
+
+  """Reword, tick or untick a checklist step"""
+  updateChecklistItem(input: UpdateChecklistItemInput!): CommandResult!
   updateIssue(input: UpdateIssueInput!): CommandResult!
   updateNotificationSettings(email: Boolean!): NotificationSettings!
   updateProject(input: UpdateProjectInput!): CommandResult!
@@ -541,6 +597,11 @@ type Query {
     limit: Int! = 50
     projectId: ID!
   ): [Activity!]!
+
+  """
+  A project's epics: top-level tickets that group others, with the progress of those
+  """
+  epics(projectId: ID!): [Epic!]!
   issue(
     """Issue id or key (e.g. "TS-42")"""
     idOrKey: String!
@@ -677,6 +738,13 @@ type Throughput {
   week: String!
 }
 
+input UpdateChecklistItemInput {
+  done: Boolean
+  issueId: ID!
+  itemId: ID!
+  text: String
+}
+
 input UpdateIssueInput {
   """An area id"""
   area: String