feat: add storage cleanup mechanism with soft delete and auto scheduler

- Add soft delete (deleted_at column) to preserve task records for statistics
- Implement cleanup service to delete old files while keeping DB records
- Add automatic cleanup scheduler (configurable interval, default 24h)
- Add admin endpoints: storage stats, cleanup trigger, scheduler status
- Update task service with admin views (include deleted/files_deleted)
- Add frontend storage management UI in admin dashboard
- Add i18n translations for storage management

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

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

backend/app/core/config.py

@@ -55,6 +55,11 @@ class Settings(BaseSettings):
     task_retention_days: int = Field(default=30)
     max_tasks_per_user: int = Field(default=1000)
 
+    # ===== Storage Cleanup Configuration =====
+    cleanup_enabled: bool = Field(default=True, description="Enable automatic file cleanup")
+    cleanup_interval_hours: int = Field(default=24, description="Hours between cleanup runs")
+    max_files_per_user: int = Field(default=50, description="Max task files to keep per user")
+
     # ===== OCR Configuration =====
     # Note: PaddleOCR models are stored in ~/.paddleocr/ and ~/.paddlex/ by default
     ocr_languages: str = Field(default="ch,en,japan,korean")