Add ASIN/ISBN fields to library and improve matching

Introduces `asin` and `isbn` fields to the PlexLibrary schema and database, with migration and indexing for fast lookups. Updates scan and recently-added processors to persist ASIN/ISBN from both Plex and Audiobookshelf backends. Enhances matching logic to prioritize exact ASIN matches using the new fields, improving match accuracy for Audiobookshelf users. Also includes minor improvements: fixes cover art handling for cached thumbnails, adds download URL validation in Prowlarr and qBittorrent integrations, and updates documentation to reflect these changes.

src/lib/processors/plex-recently-added.processor.ts

@@ -103,6 +103,8 @@ export async function processPlexRecentlyAddedCheck(payload: PlexRecentlyAddedPa
             summary: item.description,
             duration: item.duration ? item.duration * 1000 : null, // Convert seconds to milliseconds
             year: item.year,
+            asin: item.asin,  // Store ASIN from library backend
+            isbn: item.isbn,  // Store ISBN from library backend
             thumbUrl: item.coverUrl,
             plexLibraryId: libraryId!,
             addedAt: item.addedAt,
@@ -121,6 +123,8 @@ export async function processPlexRecentlyAddedCheck(payload: PlexRecentlyAddedPa
             summary: item.description || existing.summary,
             duration: item.duration ? item.duration * 1000 : existing.duration,
             year: item.year || existing.year,
+            asin: item.asin || existing.asin,  // Update ASIN if available
+            isbn: item.isbn || existing.isbn,  // Update ISBN if available
             thumbUrl: item.coverUrl || existing.thumbUrl,
             lastScannedAt: new Date(),
           },