feat(db): add imported_at column to media_items for accurate "Recently Added" sorting

The created_at column stores file modification time (intentional for preserving
original metadata), but this makes 'Recently Added' sorting unreliable for
imported files. Add imported_at column that records the actual database insert
timestamp.

Changes:
- Add imported_at TIMESTAMPTZ column to media_items (nullable)
- Update GetRecentlyAddedItems to sort by imported_at DESC NULLS LAST first
- Add ReassignLibraries and ReassignMediaItems queries for admin deletion handover
- Add SyncLibraryTypeExtensions query for startup extension sync
- Update all media_items SELECT queries to include imported_at column

database/schema/schema.sql

@@ -123,6 +123,7 @@ CREATE TABLE IF NOT EXISTS media_items (
     google_books_id VARCHAR(100), -- Google Books identifier
     added_by_admin_id UUID REFERENCES users(id) ON DELETE SET NULL,
     created_at TIMESTAMP WITH TIME ZONE DEFAULT NOW(),
+    imported_at TIMESTAMP WITH TIME ZONE DEFAULT NOW(),
     updated_at TIMESTAMP WITH TIME ZONE DEFAULT NOW(),
     -- Universal Sync Format Detection
     format_group VARCHAR(20) NOT NULL DEFAULT 'reflowable',