fix: cascade delete download_logs when video is deleted

DownloadLog has ondelete=CASCADE on FK but ORM relationship lacked
cascade='all, delete-orphan', causing IntegrityError (NOT NULL) on
video deletion.

backend/app/models.py

@@ -30,7 +30,7 @@ class Video(Base):
         Index("ix_video_url_format_id", "url", "format_id"),
     )
 
-    logs = relationship("DownloadLog", back_populates="video", lazy="select")
+    logs = relationship("DownloadLog", back_populates="video", lazy="select", cascade="all, delete-orphan")
 
 
 class DownloadLog(Base):