fix: set correct Content-Type and inline disposition for file serving

PDF was downloading instead of displaying because media_type was None
(defaulting to octet-stream). Now maps file extensions to proper MIME
types and sets Content-Disposition: inline for in-browser viewing.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

app/api/documents.py

@@ -188,10 +188,24 @@ async def get_document_file(
     if not file_path.exists():
         raise HTTPException(status_code=404, detail="파일을 찾을 수 없습니다")
 
+    # 미디어 타입 매핑
+    media_types = {
+        ".pdf": "application/pdf",
+        ".jpg": "image/jpeg", ".jpeg": "image/jpeg",
+        ".png": "image/png", ".gif": "image/gif",
+        ".bmp": "image/bmp", ".tiff": "image/tiff",
+        ".svg": "image/svg+xml",
+        ".txt": "text/plain", ".md": "text/plain",
+        ".html": "text/html", ".csv": "text/csv",
+        ".json": "application/json", ".xml": "application/xml",
+    }
+    suffix = file_path.suffix.lower()
+    media_type = media_types.get(suffix, "application/octet-stream")
+
     return FileResponse(
         path=str(file_path),
-        filename=file_path.name,
-        media_type=None,
+        media_type=media_type,
+        headers={"Content-Disposition": "inline"},
     )