feat: 完善 AI-Core 文档解析器

- 添加多种文档解析器 (PDF, Word, Excel, Markdown 等)
- 添加基础解析器和链式解析器
- 添加存储和注册机制
- 添加 gRPC 服务实现

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

ai-core/parser/image_parser.py

@@ -0,0 +1,28 @@
+import base64
+import logging
+import os
+
+from docreader.models.document import Document
+from docreader.parser.base_parser import BaseParser
+
+logger = logging.getLogger(__name__)
+
+
+class ImageParser(BaseParser):
+    """Parser for standalone image files.
+
+    Returns the image as a markdown reference with the raw image data
+    in Document.images so that the Go-side ImageResolver (or main.py's
+    _resolve_images) can handle storage upload.
+    """
+
+    def parse_into_text(self, content: bytes) -> Document:
+        logger.info("Parsing image file=%s, size=%d bytes", self.file_name, len(content))
+
+        ext = os.path.splitext(self.file_name)[1].lower() or ".png"
+        ref_path = f"images/{self.file_name}"
+
+        text = f"![{self.file_name}]({ref_path})"
+        images = {ref_path: base64.b64encode(content).decode()}
+
+        return Document(content=text, images=images)