feat: a calendar to subscribe to, and the backlog's corners

- Each person can subscribe their dates in Google Calendar, Outlook or
  Apple Calendar: GET /api/calendar/{token}.ics (no session) brings
  their unfinished tickets with a due date (assigned or followed), the
  planned and running sprints and the planned releases of their
  projects, as all-day events linking to the web. The token is made,
  renewed or dropped under /me/calendar (and in GraphQL); in the web,
  from «Calendario» in «Mi trabajo».
- The backlog panel keeps the selected rows' background inside its
  rounded corners.

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

apps/api/schema.gql

@@ -158,6 +158,15 @@ input BulkUpdateIssuesInput {
   unassign: Boolean
 }
 
+type CalendarFeed {
+  createdAt: String!
+
+  """
+  Secret of the calendar URL: GET /api/calendar/{token}.ics (iCalendar, no session)
+  """
+  token: String!
+}
+
 input ChangeIssueStatusInput {
   issueId: ID!
 
@@ -677,6 +686,9 @@ type Mutation {
   Add a rule. It acts as TaskStream, within the workflow, and what it does triggers no other rule.
   """
   createAutomation(input: CreateAutomationInput!): Automation!
+
+  """A new secret calendar URL; the previous one stops working"""
+  createCalendarFeed: CalendarFeed!
   createIssue(input: CreateIssueInput!): CommandResult!
   createProject(input: CreateProjectInput!): CommandResult!
   createRelease(input: CreateReleaseInput!): CommandResult!
@@ -737,6 +749,9 @@ type Mutation {
   """Back from the trash"""
   restoreIssue(issueId: ID!): CommandResult!
 
+  """My calendar URL stops working"""
+  revokeCalendarFeed: Boolean!
+
   """A new secret for a repository; the old one stops working"""
   rotateGitRepositorySecret(projectId: ID!, repositoryId: ID!): GitRepository!
 
@@ -996,6 +1011,9 @@ type Query {
   """The authenticated user"""
   me: Me!
 
+  """My calendar URL, if I made one"""
+  myCalendarFeed: CalendarFeed
+
   """
   Tickets across all my projects: assigned to me (default) or any (e.g. categories [QA])
   """