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/types/index.ts

@@ -10,6 +10,7 @@ export interface Library {
 
 export interface Game {
   id: string
+  item_key?: string
   title: string
   coverUrl: string | null
   wideCoverUrl: string | null
@@ -18,6 +19,7 @@ export interface Game {
 
 export interface GameSeries {
   id: string
+  item_key?: string
   title: string
   coverUrl: string | null
   wideCoverUrl: string | null
@@ -36,6 +38,7 @@ export interface FileEntry {
 
 export interface Movie {
   id: string
+  item_key?: string
   title: string
   year: number | null
   plot: string | null
@@ -49,6 +52,7 @@ export interface Movie {
 
 export interface TvSeries {
   id: string
+  item_key?: string
   title: string
   year: number | null
   plot: string | null
@@ -61,6 +65,7 @@ export interface TvSeries {
 
 export interface TvSeason {
   id: string
+  item_key?: string
   seriesId: string
   title: string
   seasonNumber: number | null
@@ -70,6 +75,7 @@ export interface TvSeason {
 
 export interface TvEpisode {
   id: string
+  item_key?: string
   title: string
   episodeNumber: number | null
   seasonNumber: number | null