GET /api/project/tickets endpoint #164

Depends on: Issue 7

List endpoint: run all three providers' list() in parallel, run the
aggregator (Issue 7), sort by max(items[].updatedAt) desc. See §6.

Acceptance criteria

• Returns Ticket[] (no discussion yet) for a given project slug
• Missing ProjectScope fields (e.g. no discordChannelId) degrade to
  that provider returning [], not an error