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/db.ts

@@ -32,9 +32,9 @@ function initDb(db: Database.Database): void {
     CREATE UNIQUE INDEX IF NOT EXISTS tags_name_category ON tags(name, category_id);
 
     CREATE TABLE IF NOT EXISTS media_tags (
-      media_key TEXT NOT NULL,
-      tag_id    TEXT NOT NULL REFERENCES tags(id) ON DELETE CASCADE,
-      PRIMARY KEY (media_key, tag_id)
+      item_key TEXT NOT NULL,
+      tag_id   TEXT NOT NULL REFERENCES tags(id) ON DELETE CASCADE,
+      PRIMARY KEY (item_key, tag_id)
     );
 
     CREATE TABLE IF NOT EXISTS libraries (
@@ -101,6 +101,7 @@ function initDb(db: Database.Database): void {
   migrateLibrariesType(db)
   migrateMediaItemsSchema(db)
   migrateMediaItemsFingerprint(db)
+  migrateMediaTagsToItemKey(db)
   seedAppSettings(db)
 }
 
@@ -177,6 +178,56 @@ function migrateMediaItemsFingerprint(db: Database.Database): void {
   }
 }
 
+function migrateMediaTagsToItemKey(db: Database.Database): void {
+  const row = db
+    .prepare("SELECT sql FROM sqlite_master WHERE type='table' AND name='media_tags'")
+    .get() as { sql: string } | undefined
+  if (!row || !row.sql.includes('media_key')) return // Already migrated or table doesn't exist
+
+  // Create replacement table with item_key column
+  db.exec(`
+    CREATE TABLE media_tags_new (
+      item_key TEXT NOT NULL,
+      tag_id   TEXT NOT NULL REFERENCES tags(id) ON DELETE CASCADE,
+      PRIMARY KEY (item_key, tag_id)
+    )
+  `)
+
+  // Build reverse mapping: short media_key → full item_key
+  // Uses same logic as the old itemKeyToMediaKey: libraryId + lastSegment
+  const items = db
+    .prepare('SELECT item_key FROM media_items')
+    .all() as { item_key: string }[]
+
+  const shortToFull: Record<string, string[]> = {}
+  for (const { item_key } of items) {
+    const firstColon = item_key.indexOf(':')
+    const lastColon = item_key.lastIndexOf(':')
+    const libraryId = item_key.slice(0, firstColon)
+    const shortId = item_key.slice(lastColon + 1)
+    const mediaKey = `${libraryId}:${shortId}`
+    ;(shortToFull[mediaKey] ??= []).push(item_key)
+  }
+
+  const tagRows = db
+    .prepare('SELECT media_key, tag_id FROM media_tags')
+    .all() as { media_key: string; tag_id: string }[]
+
+  const insert = db.prepare('INSERT OR IGNORE INTO media_tags_new (item_key, tag_id) VALUES (?, ?)')
+  db.transaction(() => {
+    for (const { media_key, tag_id } of tagRows) {
+      const candidates = shortToFull[media_key]
+      if (!candidates || candidates.length !== 1) continue // orphaned or ambiguous collision
+      insert.run(candidates[0], tag_id)
+    }
+  })()
+
+  db.exec(`
+    DROP TABLE media_tags;
+    ALTER TABLE media_tags_new RENAME TO media_tags;
+  `)
+}
+
 function migrateLibrariesType(db: Database.Database): void {
   const row = db
     .prepare("SELECT sql FROM sqlite_master WHERE type='table' AND name='libraries'")