Implement backend scan progress tracking (Steps 1-7)

Implements comprehensive progress tracking for scan jobs to provide real-time
statistics to the frontend (files_scanned, new_items, errors).

Changes:
1. Extended JobResult struct with new fields:
   - FilesScanned: total files processed
   - NewItems: books added to database
   - Errors: scan errors encountered

2. Added progress callback mechanism:
   - Job.ProgressCallback function field for real-time updates
   - Job.UpdateProgress() method to trigger callbacks
   - Worker stores callback and updates JobResult during scan

3. MediaScanner now tracks statistics:
   - totalFiles, newItems, errors counters
   - GetStats() method to retrieve statistics
   - First pass counts total files for progress calculation
   - Batches progress updates every 10 files (reduces mutex contention)
   - Final update ensures 100% progress is reported

4. Updated processMediaFile signature:
   - Returns (bool, error) instead of (error)
   - true = new item created, false = existing/updated/error
   - Increments newItems counter when creating database entries
   - Updated WatchChanges to handle new return value

5. Worker job completion extracts stats:
   - Parses result map for files_scanned, new_items, errors
   - Stores in final JobResult for API response

6. GetScanStatus API response includes new fields:
   - files_scanned, new_items, errors now in JSON response
   - Frontend can display real-time progress

Design decisions:
- Batching every 10 files balances performance vs. granularity
- Thread-safe via worker mutex (w.mu.Lock/Unlock)
- Callback pattern decouples scanner from job management
- processMediaFile return type allows tracking new vs. updated items
- Maintains backward compatibility (uses || 0 fallbacks in frontend)

Testing:
- All code compiles successfully
- Follows service layer pattern (no business logic in handlers)
- No database schema changes
- Integration tests to be added in Step 8 (separate commit)

Files modified:
- internal/services/worker.go (JobResult, Job struct, processScanJob, processJob)
- internal/services/media_scanner.go (struct fields, GetStats, ScanFolders, processMediaFile)
- internal/handlers/scanner.go (GetScanStatus response)

Related: TASKS-backend-progress-tracking.md Steps 1-7

internal/services/worker.go

@@ -28,24 +28,34 @@ const (
 )
 
 type Job struct {
-	ID          string
-	Type        JobType
-	Params      map[string]interface{}
-	Status      JobStatus
-	CreatedAt   time.Time
-	StartedAt   *time.Time
-	CompletedAt *time.Time
-	Error       error
-	Result      interface{}
-	Context     context.Context
+	ID               string
+	Type             JobType
+	Params           map[string]interface{}
+	Status           JobStatus
+	CreatedAt        time.Time
+	StartedAt        *time.Time
+	CompletedAt      *time.Time
+	Error            error
+	Result           interface{}
+	Context          context.Context
+	ProgressCallback func(progress float64, filesScanned, newItems, errors int)
+}
+
+func (j *Job) UpdateProgress(progress float64, filesScanned, newItems, errors int) {
+	if j.ProgressCallback != nil {
+		j.ProgressCallback(progress, filesScanned, newItems, errors)
+	}
 }
 
 type JobResult struct {
-	JobID    string
-	Status   JobStatus
-	Error    string
-	Result   interface{}
-	Progress float64
+	JobID        string
+	Status       JobStatus
+	Error        string
+	Result       interface{}
+	Progress     float64
+	FilesScanned int
+	NewItems     int
+	Errors       int
 }
 
 type Worker struct {
@@ -135,6 +145,15 @@ func (w *Worker) processJob(job *Job) {
 		status = JobStatusCancelled
 	}
 
+	var filesScanned, newItems, errors int
+	if result != nil {
+		if stats, ok := result.(map[string]interface{}); ok {
+			filesScanned = int(stats["files_scanned"].(float64))
+			newItems = int(stats["new_items"].(float64))
+			errors = int(stats["errors"].(float64))
+		}
+	}
+
 	w.results[job.ID] = &JobResult{
 		JobID:  job.ID,
 		Status: status,
@@ -145,8 +164,11 @@ func (w *Worker) processJob(job *Job) {
 				return ""
 			}
 		}(),
-		Result:   result,
-		Progress: 1.0,
+		Result:       result,
+		Progress:     1.0,
+		FilesScanned: filesScanned,
+		NewItems:     newItems,
+		Errors:       errors,
 	}
 	w.mu.Unlock()
 }
@@ -173,6 +195,19 @@ func (w *Worker) processScanJob(job *Job) (interface{}, error) {
 	}
 
 	scanner := NewMediaScanner(db)
+	scanner.job = job
+
+	job.ProgressCallback = func(progress float64, filesScanned, newItems, errors int) {
+		w.mu.Lock()
+		defer w.mu.Unlock()
+
+		if result, exists := w.results[job.ID]; exists {
+			result.Progress = progress
+			result.FilesScanned = filesScanned
+			result.NewItems = newItems
+			result.Errors = errors
+		}
+	}
 
 	if err := scanner.SetFolders(folders); err != nil {
 		return nil, err
@@ -188,9 +223,14 @@ func (w *Worker) processScanJob(job *Job) (interface{}, error) {
 		return nil, err
 	}
 
+	totalFiles, newItems, errors := scanner.GetStats()
+
 	return map[string]interface{}{
-		"message":    "scan completed",
-		"library_id": libraryID,
+		"message":       "scan completed",
+		"library_id":    libraryID,
+		"files_scanned": totalFiles,
+		"new_items":     newItems,
+		"errors":        errors,
 	}, nil
 }