refactor: comprehensive code quality improvements

- Add logger to all exporters for consistent error handling
- Add Course.Validate() method with ValidationError type
- Create shared test fixtures in internal/testdata
- Refactor config passing: NewArticulateParser now takes *config.Config
- Add NewLoggerFromConfig() factory for logger creation
- Add context cancellation with signal handling (SIGINT/SIGTERM)
- Add godoc examples for loggers and exporters
- Add HTML exporter error path tests
- Add fuzz tests for parser and HTMLCleaner
- Fix markdown output: trim whitespace, collapse blank lines
- Remove test output files from repo

internal/services/app_test.go

@@ -207,7 +207,7 @@ func TestApp_ProcessCourseFromFile(t *testing.T) {
 			tt.setupMocks(parser, factory, exporter)
 
 			app := NewApp(parser, factory)
-			err := app.ProcessCourseFromFile(tt.filePath, tt.format, tt.outputPath)
+			err := app.ProcessCourseFromFile(context.Background(), tt.filePath, tt.format, tt.outputPath)
 
 			if tt.expectedError != "" {
 				if err == nil {