Unify media_key and item_key — use item_key everywhere

media_key was a lossy shortening of item_key (libraryId:lastSegment) that
introduced a real collision bug: two TV episodes from different series with
the same filename would share the same media_key and each other's tags.

- DB migration converts existing media_tags rows from short format to full
  item_key by joining against media_items; ambiguous/orphaned rows are dropped
- media_tags column renamed media_key → item_key
- Removed itemKeyToMediaKey() from scanner; reconcileAndPrune now passes
  item_key directly to reKeyMediaItem
- DB reader functions (tv, movies, games) now expose item_key on returned
  entities; frontend components use entity.item_key instead of constructing
  the short libraryId:id form
- MixedView now constructs the full mixed_file: item_key format
- Tag API renamed mediaKey param → itemKey throughout

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/lib/tv.ts

@@ -215,6 +215,7 @@ export function tvSeriesFromDb(libraryId: string): TvSeries[] {
     const idPart = row.item_key.split(':tv_series:')[1] ?? row.item_key
     return {
       id: idPart,
+      item_key: row.item_key,
       title: row.title ?? decodeURIComponent(idPart),
       year: row.year ?? null,
       plot: row.plot ?? null,
@@ -242,6 +243,7 @@ export function tvSeasonsFromDb(libraryId: string, seriesId: string): TvSeason[]
       const seasonId = parts[1]?.split(':').slice(1).join(':') ?? row.item_key
       return {
         id: seasonId,
+        item_key: row.item_key,
         seriesId,
         title: row.title ?? seasonId,
         seasonNumber: meta.seasonNumber ?? null,
@@ -276,6 +278,7 @@ export function tvEpisodesFromDb(
       const episodeId = suffix.split(':').slice(2).join(':')
       return {
         id: episodeId,
+        item_key: row.item_key,
         title: row.title ?? decodeURIComponent(episodeId),
         episodeNumber: meta.episodeNumber ?? null,
         seasonNumber: meta.seasonNumber ?? null,