fix: bound backlog lazy loading

[kyui-azusa]

Summary

• scope backlog pagination checks to the backlog scroll container
• require a fresh wheel or touch gesture before adding another batch
• consume at most one 20-item batch per gesture and cap the limit at the actual backlog length

Root cause

The previous viewport-level IntersectionObserver could repeatedly observe newly rendered sentinels and append large portions of the backlog during one scroll.

Validation

• yarn webgal:build

Closes #963