refactor: remove legacy single-root APIs, defer filesystem I/O, enforce POSIX paths

- Remove legacy endpoints: /api/change-folder, /api/index, /api/scan, /api/status,
  /api/playback/resume-positions
- Remove legacy global scanner module-level API from hivescan/scanner.py
- Defer all filesystem validation to background task in server lifespan (macOS-safe)
- CLI and winmain pass raw paths via MEDIAHIVE_ROOTS; no pre-startup validation
- Enforce POSIX paths everywhere (as_posix(), no backslash leakage)
- Remove MEDIAHIVE_PATH and MEDIAHIVE_DEFER_INITIAL_ROOT env vars
- Update frontend api.ts to use per-root resume positions
- Update docs/API.md and docs/multi-index-plan.md
- Fix Python 2 style except clauses in hivescan/utils.py and scanning.py

frontend/src/components/MediaCard.vue

@@ -92,16 +92,16 @@ const posterImageUrl = computed(() => {
   if (!props.item.cover_path || isVideoPath(props.item.cover_path)) {
     return null;
   }
-  return getCoverUrl(props.item.cover_path);
+  return getCoverUrl(props.item.cover_path, props.item.root_id);
 });
 
 const posterVideoUrl = computed(() => {
   if (props.item.cover_path && isVideoPath(props.item.cover_path)) {
-    return getCoverUrl(props.item.cover_path);
+    return getCoverUrl(props.item.cover_path, props.item.root_id);
   }
 
   const fallbackVideo = props.item.showreel_images?.find(path => isVideoPath(path));
-  return fallbackVideo ? getCoverUrl(fallbackVideo) : null;
+  return fallbackVideo ? getCoverUrl(fallbackVideo, props.item.root_id) : null;
 });
 
 const rating = computed(() => {