feat: enable Markdown (.md) file upload in KB Accelerator

Moved md from Phase 2 extensions to allowed formats, added extraction
handler (reuses txt handler), and updated plan_limits defaults to
include md for all plans.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

backend/app/core/kb_extraction_service.py

@@ -159,6 +159,7 @@ def _extract_docx(content_bytes: bytes) -> ExtractResult:
 # Registry of format handlers — extend for Phase 2
 FORMAT_HANDLERS: dict[str, ExtractHandler] = {
     "txt": _extract_txt,
+    "md": _extract_txt,
     "paste": _extract_paste,
     "docx": _extract_docx,
 }