Offline: sync watch progress back to the server (D4 / op-06)

Offline playback positions stay local today. Syncing them back needs durable outbound authority, replay and conflict rules, and near-end/watched semantics for each backend (Plex, Jellyfin, Emby).

Deferred from the downloads overhaul (stack #371 → #389, integration branch downloads/overhaul). See the deferred-work section of docs/plans/2026-10-16-downloads-overhaul.md.