List the authenticated user's active channel links (#1342).

Backs the mobile on-open banner: the app reads this to decide whether to show the "you can plan routes on WhatsApp now" nudge (mobile#726), and it unblocks the point-2 resume-poll fallback (mobile#725) by letting a client that missed the silent channel_link_result push confirm link-state.

Response shape (narrow read contract — see :func:_link_state_entry)::

{
  "associations": [
    {"channel": "whatsapp", "status": "active", "phone_number": "+52..."}
  ],
  "whatsapp_linked": true,
  "notification_channel": "whatsapp",
  "linkable_channels": ["whatsapp"]
}

whatsapp_linked is the convenience flag the banner gates on, kept for builds that read it; the list is there for any channel-agnostic UI. notification_channel is where the coach's notifications go, or null for the app only (#3160). linkable_channels are the channels this deploy can mint a link code for. phone_number is WhatsApp-only; display_name is omitted (it lives on the link-request row, not the durable association).