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/metrics.go

@@ -53,6 +53,10 @@ type ScanMetrics struct {
 	// Cancelled is true when the scan was stopped via CancelScan.
 	Cancelled bool `json:"cancelled"`
 
+	// Library identification.
+	LibraryID   int64  `json:"libraryId"`   // library that was scanned
+	LibraryName string `json:"libraryName"` // display name of scanned library
+
 	// Non-fatal issues encountered during scanning.
 	Warnings []ScanWarning `json:"warnings"`
 }
@@ -60,12 +64,15 @@ type ScanMetrics struct {
 // ScanProgress is the payload emitted periodically during a scan to
 // report live progress to the frontend.
 type ScanProgress struct {
-	Phase     string `json:"phase"`     // "counting", "scanning", "orphans", "thumbnails"
-	Total     int64  `json:"total"`     // total audio files from pre-walk count
-	Processed int64  `json:"processed"` // added + skipped + updated so far
-	Added     int64  `json:"added"`
-	Skipped   int64  `json:"skipped"`
-	Updated   int64  `json:"updated"`
+	Phase       string `json:"phase"`     // "counting", "scanning", "orphans", "thumbnails"
+	Total       int64  `json:"total"`     // total audio files from pre-walk count
+	Processed   int64  `json:"processed"` // added + skipped + updated so far
+	Added       int64  `json:"added"`
+	Skipped     int64  `json:"skipped"`
+	Updated     int64  `json:"updated"`
+	LibraryID   int64  `json:"libraryId"`   // library being scanned
+	LibraryName string `json:"libraryName"` // display name of library being scanned
+	QueuedCount int    `json:"queuedCount"` // number of libraries still queued after this one
 }
 
 // ScanWarning represents a non-fatal issue encountered during scanning.