feat: 新增 .txt 纯文本文件支持

- 添加 txt 到允许上传格式，跳过 MinerU OCR
- 直接读取 UTF-8 文本，按 3000 字符/页分页后送入 LangExtract
- 适合有中文文本的规范文件，避免扫描件 OCR 乱码问题

backend/services/indexing_service.py

@@ -80,57 +80,91 @@ def _run_pipeline(job_id: str) -> None:
             _update_meta(job_id, status="cancelled", stage="Cancelled")
             return
 
-        _update_meta(job_id, status="parsing", stage="MinerU document parsing...")
-        mineru_out_dir = job_dir / "mineru_output"
-        mineru_out_dir.mkdir(parents=True, exist_ok=True)
-
-        result = subprocess.run(
-            [str(MINERU_PYTHON), str(MINERU_PIPELINE), str(pdf_path)],
-            cwd=str(MINERU_PIPELINE.parent),
-            capture_output=True,
-            text=True,
-            timeout=600,
-        )
-
-        if result.returncode != 0:
-            raise RuntimeError(f"MinerU failed: {result.stderr[:500]}")
-
-        # Find content_list.json in MinerU output
-        # MinerU writes output to mineru_mvp/output/{stem}/
-        stem = pdf_path.stem
-        mineru_default_out = MINERU_PIPELINE.parent / "output" / stem
-        content_list_path = None
-
-        if mineru_default_out.exists():
-            matches = list(mineru_default_out.glob("*_content_list.json"))
-            if matches:
-                content_list_path = matches[0]
-                # Copy to our job dir
-                import shutil
-                shutil.copytree(str(mineru_default_out), str(mineru_out_dir), dirs_exist_ok=True)
-
-        if not content_list_path:
-            # Fallback: search job mineru_output dir
-            matches = list(mineru_out_dir.glob("*_content_list.json"))
-            if matches:
-                content_list_path = matches[0]
-
-        if not content_list_path or not content_list_path.exists():
-            raise RuntimeError(f"MinerU output content_list.json not found. stdout: {result.stdout[:300]}")
-
-        # ── Stage 2: extracting ───────────────────────────────────────────
-        if _cancel_flags.get(job_id):
-            _update_meta(job_id, status="cancelled", stage="Cancelled")
-            return
-
-        from pipeline.text_assembler import load_content_list, assemble_pages, count_blocks_by_type
+        from pipeline.text_assembler import PageText, BlockSpan
         from pipeline.entity_extractor import create_model, extract_entities
         from pipeline.kg_builder import build_kg, extractions_to_records
 
-        content_list = load_content_list(content_list_path)
-        pages = assemble_pages(content_list)
-        total_pages = len(pages)
-        block_types = count_blocks_by_type(content_list)
+        ext = pdf_path.suffix.lower().lstrip(".")
+
+        if ext in ("txt",):
+            # Plain text — skip MinerU, split by paragraphs
+            _update_meta(job_id, status="parsing", stage="Parsing plain text...")
+            raw = pdf_path.read_text(encoding="utf-8")
+
+            # Split into ~3000-char pages at double-newline boundaries
+            PAGE_SIZE = 3000
+            raw_pages = []
+            pos = 0
+            while pos < len(raw):
+                end = min(pos + PAGE_SIZE, len(raw))
+                if end < len(raw):
+                    # Try to break at a double newline
+                    br = raw.rfind("\n\n", pos, end)
+                    if br > pos + PAGE_SIZE // 2:
+                        end = br + 2
+                raw_pages.append(raw[pos:end])
+                pos = end
+
+            pages = []
+            for i, text in enumerate(raw_pages):
+                pages.append(PageText(
+                    page_idx=i,
+                    text=text.strip(),
+                    block_spans=[BlockSpan(
+                        block_index=0, block_type="text",
+                        page_idx=i, char_start=0,
+                        char_end=len(text.strip()), bbox=[0, 0, 0, 0],
+                    )],
+                ))
+            total_pages = len(pages)
+            block_types = {"text": total_pages}
+
+            # Save full.txt in mineru_output for reference
+            mineru_out_dir = job_dir / "mineru_output"
+            mineru_out_dir.mkdir(parents=True, exist_ok=True)
+            (mineru_out_dir / "full.md").write_text(raw, encoding="utf-8")
+        else:
+            _update_meta(job_id, status="parsing", stage="MinerU document parsing...")
+            mineru_out_dir = job_dir / "mineru_output"
+            mineru_out_dir.mkdir(parents=True, exist_ok=True)
+
+            result = subprocess.run(
+                [str(MINERU_PYTHON), str(MINERU_PIPELINE), str(pdf_path)],
+                cwd=str(MINERU_PIPELINE.parent),
+                capture_output=True,
+                text=True,
+                timeout=600,
+            )
+
+            if result.returncode != 0:
+                raise RuntimeError(f"MinerU failed: {result.stderr[:500]}")
+
+            # Find content_list.json in MinerU output
+            from pipeline.text_assembler import load_content_list, assemble_pages, count_blocks_by_type
+
+            stem = pdf_path.stem
+            mineru_default_out = MINERU_PIPELINE.parent / "output" / stem
+            content_list_path = None
+
+            if mineru_default_out.exists():
+                matches = list(mineru_default_out.glob("*_content_list.json"))
+                if matches:
+                    content_list_path = matches[0]
+                    import shutil
+                    shutil.copytree(str(mineru_default_out), str(mineru_out_dir), dirs_exist_ok=True)
+
+            if not content_list_path:
+                matches = list(mineru_out_dir.glob("*_content_list.json"))
+                if matches:
+                    content_list_path = matches[0]
+
+            if not content_list_path or not content_list_path.exists():
+                raise RuntimeError(f"MinerU output content_list.json not found. stdout: {result.stdout[:300]}")
+
+            content_list = load_content_list(content_list_path)
+            pages = assemble_pages(content_list)
+            total_pages = len(pages)
+            block_types = count_blocks_by_type(content_list)
 
         _update_meta(
             job_id,