feat: presence on tickets

A ticket's page beats every 10 s (touchPresence / PUT
/issues/{issue}/presence, with whether I am editing) and gets back who
else is there, editing ones first. Entries live in Redis, one hash per
ticket, and fade after 30 s without a beat; leaving the page takes me out
at once. Only for people who can read the ticket; never in the event
store. The ticket shows «Bob está editando · Ana también lo está viendo»
above its title; editing means a ticket or comment form is open or a
comment is being written.

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

apps/api/schema.gql

@@ -871,6 +871,9 @@ type Mutation {
   duplicateIssue(issueId: ID!): CommandResult!
   editComment(body: String!, commentId: ID!): CommandResult!
 
+  """I left this ticket's page"""
+  leavePresence(issueId: ID!): Boolean!
+
   """Relate tickets, e.g. FE DEPENDS_ON BE, QA TESTS FE"""
   linkIssues(input: LinkIssuesInput!): CommandResult!
 
@@ -933,6 +936,11 @@ type Mutation {
   """Send a ping now and tell how it went"""
   testWebhook(webhookId: ID!): WebhookDelivery!
 
+  """
+  I am on this ticket (send every ~10 s while the page is open): answers who else is, the ones editing first
+  """
+  touchPresence(editing: Boolean! = false, issueId: ID!): [Presence!]!
+
   """
   To the trash (whoever opened it, or an owner/admin): hidden everywhere, deleted for good after 30 days
   """
@@ -1074,6 +1082,16 @@ type OptionCount {
   value: String!
 }
 
+"""Someone on a ticket right now (heartbeat in the last 30 s)"""
+type Presence {
+  """Last heartbeat"""
+  at: String!
+
+  """Editing the ticket or writing a comment"""
+  editing: Boolean!
+  user: UserRef!
+}
+
 type PriorityCount {
   count: Int!
   priority: IssuePriority!