feat: add pysubs2 subtitle conversion with extended format support

- Add pysubs2 as default conversion method for 'auto' mode
- Add support for MicroDVD (.sub), MPL2 (.mpl2), and TMP (.tmp) formats
- Implement convert_with_pysubs2() method with fallback to standard conversion
- Update from_mime() to handle new subtitle format types
- Map all supported formats to pysubs2 format identifiers
- Update docstrings to reflect new capabilities

pyproject.toml

@@ -58,6 +58,7 @@ dependencies = [
     "httpx>=0.28.1,<0.29",
     "cryptography>=45.0.0",
     "subby",
+    "pysubs2>=1.7.0,<2",
 ]
 
 [project.urls]