feat(plugin): added basic plugin

plugins/jellyfin/README.md

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+# MovieNight Jellyfin Plugin
+
+Thin Jellyfin server plugin for bridging Jellyfin playback/sync signals to the MovieNight backend.
+
+## Build
+
+```bash
+cd plugins/jellyfin/Jellyfin.Plugin.MovieNight
+dotnet publish -c Release
+```
+
+Install the published `net9.0` plugin files into the Jellyfin data directory under `plugins/MovieNight/`, then restart Jellyfin. This build targets Jellyfin `10.11.x`.
+
+## Backend Contract Used
+
+Current implemented calls:
+
+- `POST /api/integrations/jellyfin/sync`
+- `GET /api/integrations/jellyfin/sync-state`
+- `POST /api/integrations/jellyfin/events`
+
+Event requests use JSON with:
+
+- `event_id`
+- `event_type`
+- `occurred_at`
+- `jellyfin_user_id`
+- `item_id`
+- `payload_version`
+- `payload`
+
+The plugin sends `X-MovieNight-Plugin-Token` when configured. Completed Jellyfin playback stop events are sent as `playback.stopped`. Transient event push failures are retried three times with the same `event_id`.
+
+The config page test action posts a small synthetic event to `/api/integrations/jellyfin/events` and treats `200` as success and `401` as token/config failure.