Feat: support epub parsing

[seroperson]

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

• New Feature (non-breaking change which adds functionality)

To check this parser manually:

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])
"

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 46.75%. Comparing base (1db5409) to head (0bfbcbb).
⚠️ Report is 7 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #13650      +/-   ##
==========================================
- Coverage   46.93%   46.75%   -0.18%     
==========================================
  Files          45       44       -1     
  Lines        9891    10005     +114     
  Branches      112      112              
==========================================
+ Hits         4642     4678      +36     
- Misses       5231     5309      +78     
  Partials       18       18              

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[Copilot]

Pull request overview

Adds EPUB ingestion support to the parsing pipeline by introducing a dedicated DeepDoc EPUB parser and wiring it into both the “naive” chunking path and the flow Parser component.

Changes:

• Introduce RAGFlowEpubParser to extract XHTML content from EPUBs (spine-ordered, with a fallback scan) and chunk via the existing HTML parser.
• Register EPUB as a supported type in the flow parser (rag/flow/parser/parser.py) and naive chunker (rag/app/naive.py), and expose EpubParser from deepdoc.parser.
• Extend file type detection to recognize .epub, and add unit tests for EPUB parsing behavior.

Reviewed changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated 6 comments.

Show a summary per file

File	Description
deepdoc/parser/epub_parser.py	New EPUB parser implementation (spine ordering + fallback).
deepdoc/parser/__init__.py	Exports EpubParser from the parser package.
rag/flow/parser/parser.py	Adds EPUB support to flow parser configuration and invocation map.
rag/app/naive.py	Adds .epub handling branch in chunk() and imports EpubParser.
api/utils/file_utils.py	Recognizes .epub in filename_type() extension matching.
test/unit_test/deepdoc/parser/test_epub_parser.py	Adds unit tests covering EPUB parsing, ordering, and fallbacks.

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