Add optional markitdown extraction for Office/EPUB documents (#766)

Office documents were dropped server-side: .docx fell through to
"[Attached document file]", .xlsx/.pptx weren't recognized at all, and
the personal-docs RAG index only covered txt/md/json/pdf.

Wire the optional markitdown dependency (MIT, Microsoft) into both the
chat-attachment path (build_user_content) and the RAG indexer
(personal_docs), converting .docx/.xlsx/.pptx/.xls/.epub to Markdown.
It is lazy-imported with graceful fallback (mirrors src/pdf_runtime.py):
without it those formats show an "install to extract" banner and the
MIT core is unaffected. pypdf stays the default PDF path.

- src/markitdown_runtime.py: optional-dep loader + convert_to_markdown
- upload_handler: recognize Office/EPUB extensions + MIME types
- document_processor: extract Office docs in the chat else-branch
- personal_docs: index Office docs (DEFAULT_EXTENSIONS + dispatch)
- requirements-optional.txt + ACKNOWLEDGMENTS.md: pinned markitdown 0.1.5
- tests: markitdown_runtime + office index coverage

Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>

requirements-optional.txt

@@ -23,3 +23,14 @@ duckduckgo-search
 # network-served app — see ACKNOWLEDGMENTS.md. The MIT core (PDF *text*
 # extraction via pypdf) works without it; this only unlocks form-filling.
 PyMuPDF
+
+# Office / EPUB document text extraction (chat attachments + the personal-docs
+# RAG index). markitdown (MIT, Microsoft) converts .docx/.xlsx/.pptx/.xls/.epub
+# to Markdown — more token-efficient and model-legible than a raw dump. Optional
+# and lazy-imported via src/markitdown_runtime.py; without it those formats fall
+# back to a friendly "install to extract" banner and the core stays pure-MIT.
+# Extras pull mammoth/lxml/python-pptx/pandas/openpyxl/xlrd; the base also pulls
+# magika (onnxruntime), already a core dep via fastembed. We avoid the
+# [all]/Azure/audio extras (cloud + heavy). Pinned to a release >30 days old per
+# the dependency-age discussion in issue #485.
+markitdown[docx,pptx,xlsx,xls]==0.1.5