docs: add stripe subscription lifecycle spec + implementation plan

Captures the agreed design for Stripe webhook handling, 5-day grace
period with branded day-3/day-5 reminders, and Stripe Customer Portal
as the single subscription-management surface. Updates payments rules
to match and ignores .worktrees/ for isolated implementation work.

.gitignore

@@ -22,3 +22,4 @@ yarn-error.log
 /.vscode
 /.zed
 /.tmp/
+/.worktrees/