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>

src/document_processor.py

@@ -152,6 +152,44 @@ def _process_pdf(path: str) -> str:
         return f"\n\n[PDF processing failed: {str(e)}]"
 
 
+def _truncate_inline(text: str, limit: int = 15000) -> tuple[str, str]:
+    """Cap inline document text so a huge file can't blow the model's context."""
+    text = (text or "").strip()
+    if len(text) > limit:
+        return text[:limit], "\n[…truncated for inline context.]"
+    return text, ""
+
+
+def _process_office_document(path: str, display_name: str) -> str:
+    """Extract an Office/EPUB document to Markdown via the optional markitdown dep.
+
+    Falls back to a friendly banner when markitdown is unavailable or finds no
+    text, so a missing optional dependency never breaks the chat path.
+    """
+    from src.markitdown_runtime import (
+        is_markitdown_format,
+        convert_to_markdown,
+        load_markitdown,
+    )
+
+    if not is_markitdown_format(path):
+        return "\n\n[Attached document file]"
+
+    markdown = convert_to_markdown(path)
+    if markdown and markdown.strip():
+        title = os.path.splitext(os.path.basename(path))[0]
+        body, marker = _truncate_inline(markdown)
+        return f"\n\n[Document content — {title}]:\n{body}{marker}"
+
+    # No content: tell the user whether to install the optional dep or whether
+    # the document simply had no extractable text.
+    try:
+        load_markitdown()
+        return f"\n\n[Attached document: {display_name} — no extractable text found.]"
+    except RuntimeError as exc:
+        return f"\n\n[Attached document: {display_name} — {exc}]"
+
+
 def _load_vl_settings() -> dict:
     """Load admin settings from disk."""
     try:
@@ -429,7 +467,7 @@ def build_user_content(
             elif mime.startswith("text/") or _is_text_file(path):
                 extracted_text = _process_text_file(path)
             else:
-                extracted_text = "\n\n[Attached document file]"
+                extracted_text = _process_office_document(path, display_name)
 
             if content and content[0]["type"] == "text":
                 content[0]["text"] += extracted_text