fix: add result_dir configuration for task result storage

Changes:
- Add result_dir field to Settings class (default: ./storage/results)
- Add result_dir to ensure_directories() method

Fixes:
- AttributeError: 'Settings' object has no attribute 'result_dir'

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

backend/app/core/config.py

@@ -85,6 +85,7 @@ class Settings(BaseSettings):
     markdown_dir: str = Field(default="./storage/markdown")
     json_dir: str = Field(default="./storage/json")
     exports_dir: str = Field(default="./storage/exports")
+    result_dir: str = Field(default="./storage/results")
 
     # ===== PDF Generation Configuration =====
     pandoc_path: str = Field(default="/opt/homebrew/bin/pandoc")
@@ -133,6 +134,7 @@ class Settings(BaseSettings):
             self.markdown_dir,
             self.json_dir,
             self.exports_dir,
+            self.result_dir,
             self.paddleocr_model_dir,
             Path(self.log_file).parent,
         ]