Support inbox: foundation (tickets, messages, statuses)

New /app/{org}/{project}/support surface, separate from feedback. Two tables: support_tickets (id, project_id, subject, status, priority, requester_email, requester_name, assignee_id, source enum, external_message_id, created_at, last_activity_at) and support_messages (id, ticket_id, body_md, direction enum inbound|outbound, channel enum email|web|api, author_id, created_at). Status workflow: new -> open -> pending -> resolved -> closed. Inbox UI mirrors common helpdesk layout (list left, conversation right).

Out of scope: email ingestion (separate card), AI suggest-reply and ticket-to-post conversion (separate card), SLA timers, multi-mailbox routing. This card lands the data model + a usable manual inbox where admins can create tickets via web and reply.