M2: Design headless browser crawling service

## Update 2026-03-09

M1 is now deployed (2026-03-08). M2 design can begin once M1 measurements (#192, #194, #195) confirm baseline improvement and M1 features (#175, #176, #177, #178) are complete.

The render worker from M1 (PR #219) provides a foundation — M2 will build on it with per-source dynamic config, scroll depth control, and queue management.