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/handlers/scanner.go

@@ -223,11 +223,14 @@ func (h *Handler) GetScanStatus(c echo.Context) error {
 	}
 
 	return c.JSON(http.StatusOK, map[string]interface{}{
-		"job_id":   result.JobID,
-		"status":   result.Status,
-		"error":    result.Error,
-		"result":   result.Result,
-		"progress": result.Progress,
+		"job_id":        result.JobID,
+		"status":        result.Status,
+		"error":         result.Error,
+		"result":        result.Result,
+		"progress":      result.Progress,
+		"files_scanned": result.FilesScanned,
+		"new_items":     result.NewItems,
+		"errors":        result.Errors,
 	})
 }