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/models/task.py

@@ -55,6 +55,8 @@ class Task(Base):
     completed_at = Column(DateTime, nullable=True)
     file_deleted = Column(Boolean, default=False, nullable=False,
                          comment="Track if files were auto-deleted")
+    deleted_at = Column(DateTime, nullable=True, index=True,
+                       comment="Soft delete timestamp - NULL means not deleted")
 
     # Relationships
     user = relationship("User", back_populates="tasks")
@@ -79,7 +81,8 @@ class Task(Base):
             "created_at": self.created_at.isoformat() if self.created_at else None,
             "updated_at": self.updated_at.isoformat() if self.updated_at else None,
             "completed_at": self.completed_at.isoformat() if self.completed_at else None,
-            "file_deleted": self.file_deleted
+            "file_deleted": self.file_deleted,
+            "deleted_at": self.deleted_at.isoformat() if self.deleted_at else None
         }