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).

docker-compose.yml

@@ -35,6 +35,8 @@ services:
     volumes:
       - ./nginx.conf:/etc/nginx/nginx.conf:ro # 🔧 GEÄNDERT: Relativer Pfad
       - ./certs:/etc/nginx/certs:ro # 🔧 GEÄNDERT: Relativer Pfad
+      # Mount media volume so nginx can serve uploaded files
+      - media-data:/opt/infoscreen/server/media:ro
     depends_on:
       - server
       - dashboard