feat: mentions and following tickets

Mention project members in comments with @, follow and unfollow tickets,
and get told when something changes on the tickets you follow. Commenting
on a ticket starts following it.

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

apps/api/schema.gql

@@ -220,6 +220,9 @@ type Issue {
   type: String!
   updatedAt: String!
   version: Int!
+
+  """People following the ticket"""
+  watchers: [UserRef!]!
 }
 
 type IssueChange {
@@ -359,6 +362,9 @@ type Mutation {
   """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!
   updateIssue(input: UpdateIssueInput!): CommandResult!
   updateNotificationSettings(email: Boolean!): NotificationSettings!
   updateProject(input: UpdateProjectInput!): CommandResult!
@@ -368,6 +374,11 @@ type Mutation {
   """
   updateProjectSettings(projectId: ID!, settings: ProjectSettingsInput!): CommandResult!
   updateRelease(input: UpdateReleaseInput!): CommandResult!
+
+  """
+  Follow a ticket: be told when it moves, is reassigned or gets a comment
+  """
+  watchIssue(issueId: ID!): CommandResult!
 }
 
 type MyNotifications {