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/games.ts

@@ -149,6 +149,7 @@ export function gamesFromDb(libraryId: string): (Game | GameSeries)[] {
     const idPart = row.item_key.split(':game_series:')[1] ?? row.item_key
     seriesMap.set(row.item_key, {
       id: idPart,
+      item_key: row.item_key,
       title: row.title ?? decodeURIComponent(idPart),
       coverUrl: meta.coverUrl ?? null,
       wideCoverUrl: meta.wideCoverUrl ?? null,
@@ -163,6 +164,7 @@ export function gamesFromDb(libraryId: string): (Game | GameSeries)[] {
     const idPart = row.item_key.split(':game:')[1] ?? row.item_key
     const game: Game = {
       id: idPart,
+      item_key: row.item_key,
       title: row.title ?? decodeURIComponent(idPart),
       coverUrl: meta.coverUrl ?? null,
       wideCoverUrl: meta.wideCoverUrl ?? null,