docs(backend): expand recurrence sync runbook and transition docs

[cursor]

Update recurrence and change-recipe docs to cover transition-key mapping, planner/executor contracts, and practical triage steps for recurrence sync regressions introduced by the recent event processor refactor.

  

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Note

Low Risk
Low risk: documentation-only updates that don’t affect runtime behavior. The main risk is minor confusion if the described transition keys/tests drift from implementation.

Overview
Adds a recurrence sync regression triage runbook to docs/common-change-recipes.md, including how to capture the transition key from logs and a focused backend test command.

Expands docs/recurrence-handling.md with a clearer transition-key format and a documented planner/executor contract (plan fields, step ordering, Google-effect boundary), plus a step-to-executor-method mapping and checks for Google delete ID selection and the series-to-standalone recurrence-clearing guard.

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