feat(media): storage for images and audio in notes (M1)

- Table media (migration 5), files in $DATA_DIR/media/<docId>/.
- POST /documents/{id}/media (write access), POST .../media/fetch copies an
  external https image (SSRF-guarded, not in the demo), GET /media/{id}
  with range requests for readers of a note that shows the file.
- Format detected from the content (no SVG); own sandbox CSP on answers.
- Blocks point to files through their content (media:<id>), so the note
  sync needs no change; compaction removes files no block mentions any
  more after a day. Demo reset and demo start clear the folder.
- Limits MEDIA_MAX_IMAGE_MB, MEDIA_MAX_AUDIO_MB, MEDIA_QUOTA_MB,
  MEDIA_DEMO_MAX_MB.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

docker-compose.yml

@@ -38,6 +38,11 @@ services:
       - LEGAL_PRIVACY_CONTENT=${LEGAL_PRIVACY_CONTENT:-}
       - LEGAL_IMPRINT_CONTENT_FILE=${LEGAL_IMPRINT_CONTENT_FILE:-}
       - LEGAL_PRIVACY_CONTENT_FILE=${LEGAL_PRIVACY_CONTENT_FILE:-}
+      # Bilder und Audio in Notizen
+      - MEDIA_MAX_IMAGE_MB=${MEDIA_MAX_IMAGE_MB:-15}
+      - MEDIA_MAX_AUDIO_MB=${MEDIA_MAX_AUDIO_MB:-50}
+      - MEDIA_QUOTA_MB=${MEDIA_QUOTA_MB:-0}
+      - MEDIA_DEMO_MAX_MB=${MEDIA_DEMO_MAX_MB:-3}
       # Spracherkennung
       - STT_URL=${STT_URL:-}
       - STT_MODEL=${STT_MODEL:-deepdml/faster-whisper-large-v3-turbo-ct2}