Optimize episode loading to prevent full directory rescans

- Added _find_series_directory() to locate series without full rescan
- Added _scan_series_episodes() to scan only target series directory
- Modified _load_episodes() to use targeted scanning instead of anime_service.rescan()
- Added 15 comprehensive unit tests for optimization
- Performance improvement: <1s vs 30-60s for large libraries
- All tests passing (15 new tests + 14 existing background loader tests)

docs/instructions.md

@@ -122,10 +122,11 @@ For each task completed:
 All issues resolved!
 
 ### Recently Completed:
+
 - ✅ Fixed async generator exception handling in `get_optional_database_session`
 - ✅ Fixed NFO service year extraction from series names (e.g., "Series Name (2023)")
 - ✅ Added logic to skip NFO creation if NFO already exists
 - ✅ Added database update when existing NFOs are found
 - ✅ Added comprehensive unit tests for all fixes
 
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+---