feat(11-01): per-library scan pipeline with queue coordinator

Task 1: Schema, events, and progress types
- Add library_id to CreateAudioFile SQL INSERT and regenerate sqlc code
- Add LibraryScanQueued and LibraryScanQueueDrained event constants
- Regenerate TypeScript events via genevents
- Add LibraryID, LibraryName, QueuedCount to ScanProgress
- Add LibraryID, LibraryName to ScanMetrics
- Add libraryID field to importResult for threading through pipeline

Task 2: Scan queue coordinator and per-library scanning
- Create scan_queue.go with ScanLibrary(id), ScanAllLibraries()
- Add CancelCurrentScan(), CancelAllScans() for queue-aware cancellation
- FIFO scan queue with silent dedup (same library already scanning or queued)
- Refactor Scan() -> scanInternal(libraryID, libraryName, libraryPath)
- Replace GetAllAudioFiles with GetAudioFilesByLibrary for per-library loading
- Thread libraryID through DB writer to set CreateAudioFileParams.LibraryID
- drainQueue auto-starts next queued library or emits LibraryScanQueueDrained
- Pause freezes current scan AND queue
- Add GetScanQueueLength() and QueuedLibraryNames() for UI
- Mark CancelScan() and Scan() as deprecated

backend/library/scan_control.go

@@ -10,6 +10,9 @@ import (
 
 // CancelScan cancels an in-progress scan. Returns immediately;
 // scan goroutines stop at their next checkpoint.
+//
+// Deprecated: Use CancelCurrentScan or CancelAllScans for
+// queue-aware cancellation.
 func (l *Library) CancelScan() {
 	l.mu.Lock()
 	cancel := l.scanCancel