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/markitdown_runtime.py

@@ -0,0 +1,60 @@
+"""Helpers for the optional markitdown document-extraction dependency.
+
+markitdown (MIT, Microsoft) converts Office/EPUB documents to Markdown, which is
+more token-efficient and model-legible than a raw text dump. It is **optional**:
+install with `pip install -r requirements-optional.txt`. When absent, callers
+degrade gracefully (chat shows a hint; the RAG indexer skips the file) — the MIT
+core never hard-depends on it. Mirrors the optional-dependency pattern in
+`src/pdf_runtime.py`.
+"""
+
+import logging
+import os
+
+logger = logging.getLogger(__name__)
+
+MARKITDOWN_MISSING = (
+    "Office/EPUB document extraction requires markitdown. Install optional "
+    "dependencies with `pip install -r requirements-optional.txt`."
+)
+
+# Formats routed through markitdown. PDFs stay on pypdf (src/document_processor
+# and src/personal_docs); plain text/code/csv/json/markdown/html stay on the
+# cheaper built-in text path. These are the formats currently dropped entirely.
+MARKITDOWN_EXTS = frozenset({".docx", ".pptx", ".xlsx", ".xls", ".epub"})
+
+
+def is_markitdown_format(path: str) -> bool:
+    """True if the file extension is one we route through markitdown."""
+    return os.path.splitext(path)[1].lower() in MARKITDOWN_EXTS
+
+
+def load_markitdown():
+    """Return the MarkItDown class, or raise a user-facing setup hint."""
+    try:
+        from markitdown import MarkItDown  # optional dependency
+    except ImportError as exc:
+        raise RuntimeError(MARKITDOWN_MISSING) from exc
+    return MarkItDown
+
+
+def convert_to_markdown(path: str) -> str | None:
+    """Convert a document to Markdown text via markitdown.
+
+    Returns the extracted Markdown, or ``None`` if markitdown is unavailable or
+    the conversion fails — callers degrade gracefully rather than erroring.
+    """
+    try:
+        markitdown_cls = load_markitdown()
+    except RuntimeError:
+        logger.warning("markitdown not installed; cannot extract %s", path)
+        return None
+    try:
+        result = markitdown_cls().convert(path)
+        text = getattr(result, "text_content", None)
+        if text is None:
+            text = getattr(result, "markdown", None)
+        return text
+    except Exception as e:
+        logger.warning("markitdown failed to convert %s: %s", path, e)
+        return None