`GET /api/project/tickets/{key}` endpoint

[abdotop]

Depends on: Issue 7

Detail endpoint: resolve one Ticket's items, call comments(item) per
item across providers, merge into discussion: Comment[] sorted by
createdAt. See §6.

Acceptance criteria

• Returns one Ticket with a populated discussion feed combining all
  of its linked sources
• 404s cleanly for an unknown key