docs: update todo and roadmap for completed metrics backlog

Remove 8 completed metrics ideas from todo.txt backlog (keeping only
retention by acquisition channel). Update Phase 5 description in
roadmap to reflect implemented metrics.

Signed-off-by: Josephine Pfeiffer <hi@josie.lol>
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
Signed-off-by: Josephine Pfeiffer <hi@josie.lol>
