feat(video): add streamable video events & dashboard controls

Add end-to-end support for video events: server streaming, scheduler
metadata, API fields, and dashboard UI.

- Server: range-capable streaming endpoint with byte-range support.
- Scheduler: emits `video` object; best-effort HEAD probe adds
  `mime_type`, `size`, `accept_ranges`; placeholders for richer
  metadata (duration/resolution/bitrate/qualities/thumbnails).
- API/DB: accept and persist `event_media_id`, `autoplay`, `loop`,
  `volume` for video events.
- Frontend: Event modal supports video selection + playback options;
  FileManager increased upload size and client-side duration check
  (max 10 minutes).
- Docs/UX: bumped program-info, added UX-only changelog and updated
  Copilot instructions for contributors.
- Notes: metadata extraction (ffprobe), checksum persistence, and
  HLS/DASH transcoding are recommended follow-ups (separate changes).

MQTT_EVENT_PAYLOAD_GUIDE.md

@@ -122,7 +122,7 @@ Every event includes these common fields:
 
 **Note**: WebUntis events use the same payload structure as website events. The URL is fetched from system settings (`webuntis_url`) rather than being specified per-event. Clients treat `webuntis` and `website` event types identically—both display a website.
 
-#### Video Events (Future)
+#### Video Events
 
 ```json
 {
@@ -134,7 +134,7 @@ Every event includes these common fields:
   "group_id": 1,
   "video": {
     "type": "media",
-    "url": "http://server:8000/api/files/videos/123/video.mp4",
+    "url": "http://server:8000/api/eventmedia/stream/123/video.mp4",
     "autoplay": true,
     "loop": false,
     "volume": 0.8
@@ -142,6 +142,13 @@ Every event includes these common fields:
 }
 ```
 
+**Fields**:
+- `type`: Always "media" for video playback
+- `url`: Video streaming URL with range request support
+- `autoplay`: Whether to start playing automatically (default: true)
+- `loop`: Whether to loop the video (default: false)
+- `volume`: Playback volume from 0.0 to 1.0 (default: 0.8)
+
 #### Message Events (Future)
 
 ```json