DMs: build the inbox from `GET /api/dm/conversations`

[Adron]

Part of #84 — Direct Messages parity gaps

The endpoint returns one row per conversation, grouped by pairKey, newest activity first
— exactly what an inbox should render. InboxViewModel currently pages GET /api/dm, a flat
message list, and groups client-side.

• Switch the inbox to /api/dm/conversations, keeping the existing cursor-paging approach.
• Keep GET /api/dm for whatever still needs the flat list (if nothing does, drop it).
• Verify the unread counts line up with GET /api/dm/unread-count.

Tests: conversations parse; inbox renders one row per conversation; unread counts agree.

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Filed from the parity review of feature/parity-wip @ 14d7795 against interlinedlist.com (OpenAPI /api/openapi.json, 233 paths) and the public help centre.

[Adron]

Implemented in #90 (branch issue/85-dm-conversations). Closing here — review and any follow-up happens on the PR.

One open question carried into the PR: /api/dm/conversations is a bare object in the OpenAPI spec and undocumented in the help centre, so the row shape is inferred. An authenticated curl against the live endpoint would let the DTO be trimmed.