feat(scanning): add library scanner with rules engine

- Add ScanRule model with configurable conditions
- Add FileAnalyzer for ffprobe-based media analysis
- Add LibraryScanner with manual, scheduled and watcher modes
- Add LanguageDetector for audio language detection
- Support rule-based filtering with priority evaluation

backend/scanning/__init__.py

@@ -0,0 +1,11 @@
+"""Library scanning module for standalone mode."""
+from backend.scanning.models import ScanRule
+from backend.scanning.file_analyzer import FileAnalyzer, FileAnalysis
+from backend.scanning.detected_languages import DetectedLanguage
+
+__all__ = [
+    "ScanRule",
+    "FileAnalyzer",
+    "FileAnalysis",
+    "DetectedLanguage",
+]