Feat: support epub parsing (#13650)

Closes #1398

### What problem does this PR solve?

Adds native support for EPUB files. EPUB content is extracted in spine
(reading) order and parsed using the existing HTML parser. No new
dependencies required.

### Type of change

- [x] New Feature (non-breaking change which adds functionality)

To check this parser manually:

```python
uv run --python 3.12 python -c "
from deepdoc.parser import EpubParser

with open('$HOME/some_epub_book.epub', 'rb') as f:
  data = f.read()

sections = EpubParser()(None, binary=data, chunk_token_num=512)
print(f'Got {len(sections)} sections')
for i, s in enumerate(sections[:5]):
  print(f'\n--- Section {i} ---')
  print(s[:200])
"
```

deepdoc/parser/__init__.py

@@ -15,6 +15,7 @@
 #
 
 from .docx_parser import RAGFlowDocxParser as DocxParser
+from .epub_parser import RAGFlowEpubParser as EpubParser
 from .excel_parser import RAGFlowExcelParser as ExcelParser
 from .html_parser import RAGFlowHtmlParser as HtmlParser
 from .json_parser import RAGFlowJsonParser as JsonParser
@@ -29,6 +30,7 @@ __all__ = [
     "PdfParser",
     "PlainParser",
     "DocxParser",
+    "EpubParser",
     "ExcelParser",
     "PptParser",
     "HtmlParser",
@@ -37,4 +39,3 @@ __all__ = [
     "TxtParser",
     "MarkdownElementExtractor",
 ]
-