feat: custom fields per project

Text, number, date or option-list fields defined in the project settings
(archived, never removed; their type is fixed once saved). Tickets hold
them, edited in the side panel, listed under the title and filtered by
option in the list view; also over GraphQL, REST (?field=ID:value) and MCP.

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

apps/api/schema.gql

@@ -279,6 +279,9 @@ input CreateIssueInput {
   """An area id"""
   area: String
   assigneeId: ID
+
+  """Only the fields to change"""
+  customFields: [CustomFieldValueInput!]
   description: String
   dueDate: String
 
@@ -352,6 +355,47 @@ type CumulativeFlow {
   series: [FlowSeries!]!
 }
 
+"""A field the project adds to its tickets; its type never changes"""
+type CustomFieldDefinition {
+  archived: Boolean!
+  id: String!
+  name: String!
+
+  """The choices of a SELECT field"""
+  options: [String!]!
+  type: CustomFieldType!
+}
+
+input CustomFieldDefinitionInput {
+  archived: Boolean! = false
+  id: String!
+  name: String!
+
+  """The choices of a SELECT field"""
+  options: [String!]! = []
+  type: CustomFieldType!
+}
+
+"""What a custom field holds (DATE: YYYY-MM-DD)"""
+enum CustomFieldType {
+  DATE
+  NUMBER
+  SELECT
+  TEXT
+}
+
+"""A ticket's value of a field"""
+type CustomFieldValue {
+  fieldId: String!
+  value: String!
+}
+
+"""A custom field's value; null or '' clears it"""
+input CustomFieldValueInput {
+  fieldId: String!
+  value: String
+}
+
 """A commit, branch or pull request that mentions the ticket"""
 type DevLink {
   at: String!
@@ -443,6 +487,9 @@ type Issue {
   commentCount: Int!
   comments(limit: Int! = 50, offset: Int! = 0): CommentPage!
   createdAt: String!
+
+  """Values of the project's custom fields (only those set)"""
+  customFields: [CustomFieldValue!]!
   description: String
 
   """Commits, branches and pull requests that mention it"""
@@ -516,6 +563,9 @@ input IssueFilterInput {
   assigneeId: ID
   categories: [StatusCategory!]
 
+  """Only tickets holding these custom values"""
+  customFields: [CustomFieldValueInput!]
+
   """Due on or after this day (YYYY-MM-DD)"""
   dueFrom: String
 
@@ -961,9 +1011,12 @@ enum ProjectRole {
   VIEWER
 }
 
-"""Statuses (board order), transitions, ticket types and areas"""
+"""
+Statuses (board order), transitions, ticket types, areas and custom fields
+"""
 type ProjectSettings {
   areas: [AreaOption!]!
+  customFields: [CustomFieldDefinition!]!
   issueTypes: [IssueTypeOption!]!
   statuses: [WorkflowStatus!]!
   transitions: [StatusTransitions!]!
@@ -974,6 +1027,9 @@ The whole configuration (replaces the previous one). Statuses go in board order.
 """
 input ProjectSettingsInput {
   areas: [AreaOptionInput!]!
+
+  """Absent: unchanged. Fields cannot be removed: archive them."""
+  customFields: [CustomFieldDefinitionInput!]
   issueTypes: [IssueTypeOptionInput!]!
   statuses: [WorkflowStatusInput!]!
   transitions: [StatusTransitionsInput!]!
@@ -1327,6 +1383,9 @@ input UpdateChecklistItemInput {
 input UpdateIssueInput {
   """An area id"""
   area: String
+
+  """Only the fields to change"""
+  customFields: [CustomFieldValueInput!]
   description: String
   dueDate: String