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/scanner.ts

@@ -556,22 +556,6 @@ function detectMoves(
  * Tags on deleted items are intentionally left as orphans — harmless and
  * recoverable if the file reappears.
  */
-/**
- * Converts an item_key (used in media_items) to the media_key format used in
- * media_tags. The UI constructs media_keys as `${libraryId}:${shortId}` where
- * shortId is only the terminal path segment — e.g.:
- *   "lib1:movie:Inception%20(2010)"     → "lib1:Inception%20(2010)"
- *   "lib1:tv_episode:Show:S1:ep.mkv"   → "lib1:ep.mkv"
- *   "lib1:mixed_file:dir%2Ffile.mp4"   → "lib1:dir%2Ffile.mp4"
- */
-function itemKeyToMediaKey(itemKey: string): string {
-  const firstColon = itemKey.indexOf(':')
-  const lastColon = itemKey.lastIndexOf(':')
-  const libraryId = itemKey.slice(0, firstColon)
-  const shortId = itemKey.slice(lastColon + 1)
-  return `${libraryId}:${shortId}`
-}
-
 function reconcileAndPrune(
   db: Database.Database,
   libraryId: string,
@@ -583,11 +567,8 @@ function reconcileAndPrune(
   // Apply moves first (outside transaction so console.log is visible as they happen)
   for (const { oldKey, newKey } of moves) {
     renameItem.run(newKey, oldKey)
-    // Convert item_keys to the media_key format actually used in media_tags
-    const oldMediaKey = itemKeyToMediaKey(oldKey)
-    const newMediaKey = itemKeyToMediaKey(newKey)
-    if (oldMediaKey !== newMediaKey) {
-      reKeyMediaItem(oldMediaKey, newMediaKey)
+    if (oldKey !== newKey) {
+      reKeyMediaItem(oldKey, newKey)
     }
     console.log(`[scanner]   fingerprint match: renamed "${oldKey}" → "${newKey}"`)
   }