feat: archive a project instead of deleting it

The owner archives a finished project (archiveProject, POST
/projects/{project}/archive) and brings it back (unarchive). While
archived it stays readable, managers still read its rules and webhooks,
but no command changes it: contributing and managing checks refuse with
422, votes and public proposals too, and automations do not run. The web
shows a read-only notice on every page of it (with «Desarchivar» for the
owner), hides what edits, marks it on the home page and lists it last.

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

apps/api/schema.gql

@@ -792,6 +792,11 @@ type Mutation {
 
   """Add a Pireon user to the project"""
   addProjectMember(input: ProjectMemberInput!): CommandResult!
+
+  """
+  Archive the project (owner only): everything stays readable, nothing changes until it is unarchived
+  """
+  archiveProject(projectId: ID!): CommandResult!
   archiveRelease(
     """
     Open release of the same project that takes its unfinished tickets; none: they leave it
@@ -923,6 +928,9 @@ type Mutation {
   To the trash (whoever opened it, or an owner/admin): hidden everywhere, deleted for good after 30 days
   """
   trashIssue(issueId: ID!): CommandResult!
+
+  """Bring an archived project back (owner only)"""
+  unarchiveProject(projectId: ID!): CommandResult!
   unlinkIssues(input: LinkIssuesInput!): CommandResult!
 
   """Push off on this device"""
@@ -1071,6 +1079,9 @@ type Project {
   """
   analytics(days: Int! = 90): ProjectAnalytics!
 
+  """Archived since then (read-only); null while active"""
+  archivedAt: String
+
   """Kanban board, one column per status"""
   board(
     area: String